BN nanotubes in ropes grown by a continuous laser heating
Physics Letters, Vol. 76, 22, p. 3239, 29 may 2000
(Reprints are available on demand)
A continuous CO2 laser
beamwas focused on a surface of hexagonal boron nitride under low nitrogen
pressure, inducing a stiff radial temperature gradient, rising locally
over the dissociation temperature. Boron nitride nanotubes and faceted
boron nitride onions were found on the heated surface, segregated in
specific zones. Nanotubes were grown in macroscopic quantity,
stoichiometric , well crystallized, mostly with few layers
and self-assembled in ropes as long as 40 microns. The cavity of
boron nitride onions was often occupied by a boron nano-crystal. The recombination
of boron and nitrogen both in plasma and on surface is suggested
for the nucleation process.
(a) Surface after heating (b) SEM
view of zone III
380 k jpeg
|Fig. 1: (a) Growth on the heated
surface. (Binocular microscope image.) The heating has resulted in segregation
in four zones. I: Drops of solidified boron. II: h-BN platelets. III (75
microns large ring): Thin BN nanotubes have grown self-assembled in ropes
forming a crown perpendicular to the surface. The most developed part of
the crown (up image) is close to a large boron drop in the hole (arrow).
IV: The surface is covered by BN nano-polyhedrons. Diameter of the hole:0.55
mm. (b) SEM view of the crown-like material (corresponds to Fig. 1a, ring
III, up image). Tangled BN ropes, as long as 40 microns, are mixed with
BN nano-polyhedrons powders. Scale bar 10 microns.
TEM imaging of BN tubes.
|Fig. 2: (a) Curl of a rope.
A section of a rope, which is perpendicular to the electron beam, appears
contrasted. It contains ~ 10 tubes. During the imaging, the rope is slightly
vibrating due to the charging effect under the electron beam. Scale bar
10 nm. (b) Section image of a three-layer tube showing a circular section.
Scale bar 2 nm. (c) Two-layer tube. Scale bar 2 nm.
|Fig. 3: On a BN rope (beam focused
to the diameter of the rope ~ 40 nm), the inelastic electron energy loss
is very similar to that of (stoichiometric) h-BN. For a large (50 nm) BN
nano-polyhedrons containing a boron nano-crystal (20 nm probe), the N/B
ration is non-stoichiometric. Spectra have been background corrected, vertically
expended to an equal N-K loss and vertically shifted of an arbitrary offset.
BN onion of crystalline B core
|Fig. 4: TEM imaging of a BN nano-polyhedron
containing a boron nano-crystal (direction 0001, hexagonal indexing). New
layers of BN seem to be emerging from the boron lattice (arrow) in a recombination
process. Scale bar 10 nm.