Hexagonal boron nitride (h-BN) is a layered material with a graphite-type
structure in which planar networks of BN hexagons are regularly stacked. As a
structural analogue of carbon nanotubes (CNTs) a BN nanotube (BNNT) was firstly
predicted in 19941 and synthesized the next
year.2 Since then, it has become one of the most
intriguing non-carbon nanostructures.3
Compared with metallic or semiconducting CNTs, a BNNT is an electrical
insulator with a bandgap of ca. 5.8 eV, basically independent of tube geometry.
BNNTs possess superb chemical and thermal stability, excellent mechanical
properties, and high thermal conductivity. Such unique features make these tubes
promising in a variety of fields such as nanodevices, functional composites,
hydrogen accumulators, etc.
In addition, recent breakthroughs in studying a new C form - graphene - a
single layer of graphite, have brought to the research forefront the questions
of existence and stability of its BN analog - a BN nanosheet, Figure 1.
Figure 1. Structural
models of a BN monoatomic sheet and a single-shelled BN nanotube. Alternating B
and N atoms are shown in blue and red.
Significant progress has been achieved within our group at NIMS regarding the
synthesis, structural analysis and property measurements of multi-walled BNNTs
and nanosheets. The nanotubes are currently synthesized in gram level quantities
in a single run, Figure 2.4 The BN nanosheets were
also prepared at a high yield under hBN ultrasonication.5
|Figure 2. (a)
Synthesis of multi-walled BN nanotubes using MgO, B and FeO precursors in a
vertical induction furnace at ~1500°C;4 (b) Photo
images of a BNNT product; (c, d) SEM and TEM images; (e) Histograms of the
external tube diameter distribution.
Electrical and mechanical properties of the tubes/sheets were studied in a
transmission electron microscope (TEM) and an atomic force microscope (AFM). The
Young's modulus of nanotubes was measured as high as ~0.8 TPa, Figure 3,6 whereas the bending modulus of BN nanosheets reached ~30
GPa, Figure 4.7
|Figure 3.(a,b) Images
of a multi-walled BN nanotube under its bending and reloading in TEM. The tube
fully restores its original shape due to its superb elasticity and flexibility.
The insets in (a,b) show the same tube at a lower-magnification; c) A sketch
showing a designed deformation experiment inside TEM using a Si AFM cantilever.
The force-displacement curve is recorded in-tandem with TEM imaging.6
|Figure 4. (a) AFM
topography image of a Ti/Au contacts-clamped BN nanosheet placed under the
trench of a Si/SiO2 substrate; and (b) The measured bending modulus
of BN nanosheets as a function of their dimensions.7
The performance of BNNTs in a high electrical bias regime was also
analyzed.8,9 Due to the ioinicity
of a B-N bond, the BN tube decomposition temperature was found to be
proportional to an applied electrical field, Figure 5.
|Figure 5. a) An
individual multi-walled BN nanotube placed between two metal contacts in STM-TEM
and biased to ~140 V.8 The appearance of amorphous
B-balls (arrowed) under tube chemical decomposition is apparent. B and N
elemental maps of a decomposed tube are shown in the insets; b) Color-coded map
of kinetic energy of atoms in a BN nanotube placed in an electrical field at
certain temperature.9 Experimental data (circles)
spread around an equipotential line of 0.165 eV. The line hits 1910 K (normal
temperature of BN thermal decomposition) at a zero electrical
In summary, it should be emphasized that the rich application potentials of
BN nanotubes and nanosheets, that have long been in the shadow of their popular
C counterparts, have largely been underestimated and deserve more detailed
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Copyright AZoNano.com, Professor Dmitri Golberg (National
Institute for Materials Science (NIMS))