Patterned growth and cathodoluminescence of conical boron nitride nanorods

Article abstract of DOI:10.1063/1.2179144

We demonstrate a simple and effective approach for growing large-scale, high-density, and well-patterned conical boron nitride nanorods. A catalyst layer of Fe (NO3) 3 was patterned on a silicon substrate by using a copper grid as a mask. The nanorods were grown via annealing milled boron carbide powders at 1300 °C in a flow of nitrogen gas. The as-grown nanorods exhibit uniform morphology and the catalyst pattern precisely defines the position of nanorod deposition. Cathodoluminescence (CL) spectra of the nanorods show two broad emission bands centered at 3.75 and 1.85 eV. Panchromatic CL images reveal clear patterned structure.

Full text of DOI:10.1063/1.2179144

Patterned growth and cathodoluminescence of conical boron nitride nanorods
H. Z. Zhang, M. R. Phillips, J. D. Fitz Gerald, J. Yu, and Y. Chen
Citation: Applied Physics Letters 88, 093117 (2006); doi: 10.1063/1.2179144
View online: http://dx.doi.org/10.1063/1.2179144
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APPLIED PHYSICS LETTERS 88, 093117 ͑2006͒
Patterned growth and cathodoluminescence of conical boron
nitride nanorods
H. Z. Zhanga͒
Department of Electronic Materials Engineering, Research School of Physical Science and Engineering,
The Australian National University, Canberra, ACT 0200, Australia
M. R. Phillips
UTS Institute of Nanoscale Technology, University of Technology, Sydney, NSW 2007, Australia
J. D. Fitz Gerald
Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
J. Yu and Y. Chen
Department of Electrorric Materials Engineering, Research School of Physical Science and Engineering,
The Australian National University, Canberra, ACT 0200, Australia
͑Received 21 June 2005; accepted 11 January 2006; published online 2 March 2006͒
We demonstrate a simple and effective approach for growing large-scale, high-density, and
well-patterned conical boron nitride nanorods. A catalyst layer of Fe͑NO3͒3 was patterned on a
silicon substrate by using a copper grid as a mask. The nanorods were grown via annealing milled
boron carbide powders at 1300 °C in a flow of nitrogen gas. The as-grown nanorods exhibit uniform
morphology and the catalyst pattern precisely defines the position of nanorod deposition.
Cathodoluminescence ͑CL͒ spectra of the nanorods show two broad emission bands centered at 3.75
and 1.85 eV. Panchromatic CL images reveal clear patterned structure. © 2006 American Institute
of Physics. ͓DOI: 10.1063/1.2179144͔
Boron nitride is an isoelectronic analog of carbon and
exhibits at least four polymorphic modifications: hexagonal
͑hBN͒, rhombohedral ͑rBN͒, zinc blende ͑cBN͒, and wurtz-
itic ͑wBN͒.1 Because of their good mechanical, thermal, and
chemical properties, BN powders and BN thin films have
been widely used as refractory materials, lubricants, abrasive
grains, and hard coatings. BN is also an electrical insulator,
for example, the band gap of cBN is larger than 6.4 eV while
measurements of the band gap of hBN span a wide range
from 3.6 to 7.1 eV.2,3 The large band gap makes boron ni-
material,7,8 and one-dimensional BN nanomaterials with dif-
ferent structures and dimensions may well have different
emission behaviors. Recently, conical boron nitride nanorods
with a mean diameter of 60 nm have been synthesized by
annealing ball-milled boron carbide.20 The unusual conical
structure of these BN nanorods makes their luminescence
properties an interesting topic. In addition, to our knowledge
selective growth of one-dimensional BN nanomaterials, spe-
cifically patterned growth, has not been realized, which
could become a hindrance for their further applications. In
this letter, we report a simple and effective approach for
patterned growth of the conical boron nitride nanorods and
describe their cathodoluminescence properties.
Ball milling and annealing was used to grow the conical
boron nitride nanorods, as reported elsewhere, milled boron
carbide powders were annealed in a flowing nitrogen
atmosphere.20–22 To grow the nanorods selectively on a sili-
con substrate, i.e., patterned growth, a catalyst layer of
Fe͑NO3͒3 was first deposited on the substrate as follows. A
copper grid ͑3 mm in diameter, square mesh with a width of
50 ␮m͒ was clipped onto the polished surface of a silicon
substrate. The temperature of the substrate and the grid was
then kept at about 100 °C. Several drops ͑ϳ0.1 ml͒ of
Fe͑NO3͒3 in ethanol ͑1.2ϫ10−3 mol/l͒ were placed on the
grid. At the substrate temperature of ϳ100 °C, the ethanol
evaporates at a rate that allows the solution to first fill the
holes of the grid then evaporate without penetrating into the
area shielded by the copper grid to any extent. After several
minutes of additional heating to dry up ethanol, the copper
grid was lifted off the substrate, leaving a well-defined pat-
tern of catalyst. This substrate was inverted and placed on
the top of milled B4C powders directly, and annealed at
1300 °C for 8 h in a tube furnace with a flow of nitrogen of
ϳ1.4 l/min. The morphology of the substrate surface was
tride
a promising candidate for deep-blue and UV
applications.4 Its luminescence properties have been thus
studied extensively, and a number of luminescence centers
have been reported.3,5–9 An ultraviolet light-emitting diode of
a cubic boron nitride pn junction has been realized,10 and
room-temperature ultraviolet lasing has been observed in an
hBN single crystal.11 Quasi-one-dimensional boron nitride
nanomaterials such as BN nanotubes,12 BN nanocones,13 BN
helixes,14 and BN whiskers,15 have been investigated exten-
sively. In contrast to carbon nanotubes, BN nanotubes have
uniform electronic band gaps, independent of their diameters
and chiralities.16 Quantum confinement effects in these low-
dimensional materials can enhance their optical emission
substantially by inducing an indirect-to-direct conversion of
the optical transition.17,18 Therefore, one-dimensional BN na-
nomaterials are likely to find further applications in optoelec-
tronics, but only a few reports exist about the luminescence
properties of one-dimensional BN nanomaterials.15,19 Photo-
luminescence and cathodoluminescence of large BN whis-
kers consisting of nanofiber-interweaved network have been
studied.15,19 It is well known that impurities or defects can
introduce acceptor or donor levels in the band gap of a
a͒
Electronic mail: hongzhou zhang@anu.edu.au
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0003-6951/2006/88͑9͒/093117/3/$23.00 88, 093117-1 © 2006 American Institute of Physics
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093117-2
Zhang et al.
Appl. Phys. Lett. 88, 093117 ͑2006͒
FIG. 1. SEM images of the as-grown conical nanorod deposits: ͑a͒ low-
magnification image showing the patterned growth, the inset shows a well-
defined boundary between the growth and nongrowth regions, and ͑b͒ high-
magnification image displaying the typical morphology of the nanorods.
FIG. 2. ͑a͒ Bright-field TEM image shows the nanorods have straight rod-
like morphology and catalyst particles are indicated by the black arrows; the
inset is the selective area electron diffraction pattern from an individual
nanorod and two rows of 000l can be discerned. ͑b͒ The detail of one
nanorod shows typical conical nature with voids ͑arrowed͒ existing in the
center of the nanorods, and magnified in the inset to show BN lattice fringes.
The speckled feature in the background marked with a series of “c”s is the
lacey-carbon film supporting the sample for TEM.
observed with a field emission scanning electron microscope
͑FESEM͒ ͑Hitachi 4500͒. The structure of the nanorods were
studied by using a transmission electron microscope ͑Philips
CM300͒ working at 300 kV. The composition of the nano-
rods was identified by using an energy-dispersive x-ray spec-
trometer ͑EDX͒, fitted with a superultrathin window. Cathod-
oluminescence ͑CL͒ spectra of the sample were measured by
using a JEOL 35C FESEM ͑the accelerating voltage and
beam current was 20 kV and 100 nA, respectively͒ equipped
with an Oxford Instruments MonoCL2 and CL spectra were
collected at both 300 and 80 K using a liquid nitrogen cold
stage. All CL spectra have been corrected for the response of
the collection system. Panchromatic CL images of the pattern
were taken by using an off-axis mirror and Hitachi CL de-
tector in a Hitachi S2250-N.
The typical morphology of the as-grown conical boron
nitride nanorod deposit is shown in the SEM image of Fig.
1͑a͒. Large quantities of nanorods grew on the 50 ␮m blocks
corresponding to the squares of the copper grid, the pattern
being produced uniformly in a macroscopic scale of 3 mm. It
is expected that patterns with different configurations and
larger areas can be achieved by using this simple method.
The inset of Fig. 1͑a͒ indicates the well-defined boundaries
of the deposits. Figure 1͑b͒ reveals the morphologies of in-
dividual nanorods. The lengths of the nanorods can be up to
ϳ10 ␮m, and their diameters are around 60 nm on average,
and they are randomly oriented.
EDX results show that the nanorods are composed of only
boron and nitrogen.
Figure 3 are CL spectra of the specimen measured with a
scan area of 17 ␮mϫ14 ␮m, and spectra in Fig. 3͑a͒ were
collected from the same region of specimen and spectra in
Fig. 3͑b͒ were collected from the same region but a different
analysis area to Fig. 3͑a͒. All the CL spectra have two broad-
bands: an ultraviolet band and a red band centered at about
ϳ3.8 and ϳ1.8 eV, respectively. A band structure calculation
taking into account interlayer interaction proposes that hBN
exibits an indirect gap of 3.9 eV and the UV band of the
nanorods could be the near-band-edge emission.2,3 However,
values of hBN band gap in literature reports have a wide
range24,25 and often extend outside the detection range of the
CL system used in this work. Consequently, it is possible that
the origin of the observed 3.8 eV emission can be attributed
to defect-related centers instead of band-to-band transitions.
In this case, the red shift of the 3.8 eV peak with increasing
specimen temperature could result from the presence of two
centers at 3.4 and 3.8 eVeach with a different thermal behav-
ior rather than an increase in the band gap due to lattice
expansion. The temperature and excitation density depen-
dence of the 3.8 eV peak ͓shown in Figs. 3͑a͒ and 3͑b͒,
respectively͔ are consistent with a shallow donor-deep ac-
ceptor pair ͑DAP͒. The blue shift of the 3.8 eV band with
increasing beam current can be attributed to saturation of
more distant donor-acceptor pairs while the decrease in
3.8 eV intensity with increasing temperature can be ex-
plained by thermal ionization of the shallow donor involved
in the DAP. The decrease in the 3.8 eV vs 1.8 eV intensity
ratio with increasing beam current ͓Fig. 3͑b͔͒ indicates that
the 3.8 eV emission is saturating with respect to the 1.8 eV.
This result also suggests that the 3.8 eV peak is defect re-
lated because saturation behavior is not expected for band-
to-band transitions.
Transmission electron microscopy ͑TEM͒ analyses re-
veal more details of the structure of the as-grown nanorods.
As shown in Fig. 2͑a͒, the nanorods typically exhibit
straight-rod morphology. A tip particle, which is the Fe–Si
alloy enclosed by BN layers, can be often observed on one
end of a nanorod ͓black arrows in Fig. 2͑a͔͒. The inset of
Fig. 2͑a͒ is a selected-area electron diffraction pattern taken
from a single nanorod. Two rows of 000l reflection spots as
well as 10.0 reflection loci can be discerned. This indicates
that the incident electron beam was more or less perpendicu-
lar to the axis of the nanorod and suggests a conical mor-
phology of the nanorod.13,23 The apex angle of the nanorod
can be estimated by the interangle of the two sets of 000l
spots. Lattice image, Fig. 2͑b͒, shows the conical nature of
the nanorods clearly. The inclination of the 000l fringes ͑see
the inset͒ and the nanorod axis is clearly seen. The distribu-
tion of the apex angle of the nanorods does not support any
nonhexagonal member rings, which, according to some re-
searchers, is evidence of a helical structure.13,23 However, as
shown in Fig. 2͑b͒, voids exist at the cores of the nanorods
alternating with lattice fringes which extend continuously
from one side of the nanorod to the other side. The existence
of these voids cannot be explained with a helical model, and
The origin of the red band, which has been observed in
cBN and boron nitride whiskers, is also controversial.15 For
the cBN, vacancy-type defect and dislocation bands are re-
sponsible for the emission, while sp3-bonded structures and
size confinement have been considered in the whisker case.
As shown in Fig. 2͑b͒, to form the cup-like morphology,
atomic configurations in the nanorods must have a number of
nonhexagonal rings around the voids where vacancy-type de-
fect and dislocation may well be introduced. On the surfaces
of the nanorods, the BN cones within each stack are inter-
connected through more or less sp3-hybridized bonds. Con-
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it is more likely that the nanorods consist of stacked cones.
sidering the small dimensions and thus the large surface-to-
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093117-3
Zhang et al.
Appl. Phys. Lett. 88, 093117 ͑2006͒
FIG. 4. Panchromatic CL images ͑a͒ showing the square pattern structure
inclined ϳ40° to the image edges ͑b͒ strong emission from the catalyst
region.
core region. Two emission bands centered at ϳ3.8 and
ϳ1.8 eV were observed in their cathodoluminescence spec-
tra. CL images display clear pattern structures. The selective
growth and the strong CL emission of the nanorods suggest
promising UV applications of the nanorods.
The authors appreciate Dr. Sally Stowe for helpful dis-
cussions and assistance with the SEM and CL, and they
thank Dr. Cheng Huang for assistance with the SEM. This
work is financially supported, in part, by The Australian Re-
search Council under the nanotube program of the Center for
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