Single-crystalline CeB6 nanowires

Article abstract of DOI:10.1021/ja051340t

Considered to be one of the best electron emission materials, CeB₆ nanowires are of great scientific and industrial interest. CeB₆ nanowires of square cross section with about 50 nm in lateral dimension and more than several micrometers in length have been successfully produced using a chemical vapor deposition method. The nanowires are grown in the <100> lattice direction, and both the tip top and the side surfaces are terminated with the {100} lattice planes. Copyright

Full text of DOI:10.1021/ja051340t

Published on Web 05/12/2005
Single-Crystalline CeB6 Nanowires
Han Zhang,† Qi Zhang,‡ Jie Tang,‡,§ and Lu-Chang Qin*,†,‡
Curriculum in Applied and Materials Sciences and Department of Physics and Astronomy, UniVersity of North
Carolina at Chapel Hill Chapel Hill, North Carolina 27599-3255, and National Institute for Materials Science,
Tsukuba, Japan
Received March 3, 2005; E-mail: lcqin@physics.unc.edu
Single-crystalline rare-earth hexaborides, especially lanthanum
hexaboride (LaB6) and cerium hexaboride (CeB6), are thermionic
cathode materials widely used in a broad range of electron beam
applications where a high brightness electron source with long
service life is required. The advantage of using rare-earth hexaboride
originated from its crystal structure, which is composed of rare-
earth metal atoms embedded inside a stable boron octahedron
network.1 This arrangement allows a unique combination of all the
desired properties for an excellent cathode material, such as low
work function, low volatility, low electrical resistivity, high
mechanical strength, and high chemical resistance, while these
properties seldom coexist in any other materials. Compared to LaB6,
CeB6 is believed to have even lower work function (∼2.5 eV) and
lower volatility, which mean lower operation temperature and longer
service life when used as a thermionic electron emitter.2,3 The low
work function is even more crucial a criterion when a cathode
material is designed for field-induced electron emission applications,
which offer a brightness 100 or more times higher than a thermionic
electron source. Field emission current density J can be expressed
by the Fowler-Nordheim equation:4
J ) 1.5 × 10-6(E2/φ) exp(10.4/φ1/2 - 6.44 ×
107φ3/2/E)A/cm2
Figure 1. (a) Low-magnification TEM image of a CeB6 nanowire. (b-d)
Electron diffraction patterns of the same CeB6 nanowire tilted to zone axes
[100], [11h0], and [21h0], respectively. Dashed lines indicate the tilting axes
about which the nanowire is rotated to the desired orientation. Circular
where E is the local electric field produced at the tip and φ is the
work function of the emitting surface of the tip. It can be inferred
that the emission current density J is enhanced almost exponentially
with the decrease of work function φ. However, to achieve a high
emission current density J, high local electric field E is another
important factor and can be expressed as E ) V/5r, with V being
the voltage applied on the emitter tip and r being the tip radius.
The difficulty in making single-crystalline rare-earth hexaboride
sharp tips has impeded the development of field emission cathode
from this low work function material. Several attempts were made
to fabricate electrolytically sharpened LaB6 and CeB6 needle-shaped
field emitters from bulk single crystals, but no satisfactory field
emission results have been reported, in contrast to the highly
expected low turn-on voltage and high emission current promised
by the low work function that these hexaborides possess.5-9 On
the other hand, sharper tips and higher aspect ratio will also enhance
field emission for a given material. In a recent communication, we
reported a successful synthesis of 111 oriented LaB6 nanowires
with diameter below 100 nm.10 In a subsequent article, we reported
the fabrication of 001 oriented LaB6 nanowires and measurement
of their field emission characteristics where a field emission current
density of about 5 × 105 A/cm2 was obtained at a driving voltage
of 800 V.11 In this communication, we present the successful
arrows indicate the tilting angles, and the straight arrows point to the tilting
sequence in experiment.
synthesis of single-crystalline CeB6 nanowires with lateral dimen-
sion around 50 nm, length more than several micrometers, and a
well-defined 001 crystal growth direction.
The synthesis is based on the following chemical reaction:
2CeCl3(g) + 12BCl3(g) + 21H2(g) ) 2CeB6(s) + 42HCl(g)
The reaction was conducted in a tube furnace operated at 1125 °C
where the BCl3 gas (Aldrich, 99.9%) was introduced to the reaction
zone in a quartz tube and CeCl3 powders (Aldrich, 99.9%) were
vaporized in a quartz boat placed inside the same tube furnace. A
silicon substrate coated with platinum was placed at a position
downstream of the quartz boat for CeB6 nanowire deposition.
After the reaction, CeB6 nanowires with lateral dimensions
ranging from below 50 nm to more than 1 µm were grown on the
substrate surface. The nanowires were examined in both scanning
electron microscope (SEM, JEM-6300) equipped with an energy-
dispersive X-ray spectrometer (EDX, KevexSigma3) and transmis-
sion electron microscope (JEM-2010F operated at 200 kV). EDX
analysis indicates that these nanowires are composed of B and Ce
elements.
† Curriculum in Applied and Materials Sciences, University of North Carolina.
‡ Department of Physics and Astronomy, University of North Carolina.
§ National Institute for Materials Science.
A representative morphological study and structure determination
of the nanowires are presented in Figure 1. Figure 1a is a typical
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10.1021/ja051340t CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
TEM image showing that the CeB6 nanowire has a flat tip top, and
its lateral dimension is measured to be 52 nm in this orientation. A
high-resolution electron microscopy (HRTEM) image of the top
left corner the CeB6 nanowire is displayed in the inset. We found
that, similar to the LaB6 nanowires,11 the growth direction is the
[001] axis of the crystal and both the flat tip top and the nanowire
side surfaces are terminated with the {100} lattice planes. Figure
2b is a TEM image of the same tip after a 45° tilting about the
axial direction of the nanowire. As expected, the electron diffraction
pattern confirmed that the nanowire was oriented in the [11h0] zone
axis, as also shown in the HRTEM image in Figure 2b. Moreover,
since now the projected width of the nanowire at this angle is 73
x
nm, which makes a ratio of 2 with that obtained in Figure 2a, it
suggests that the nanowire has a square cross section. This
morphology of the nanowire is also indicated in the left image of
Figure 2b, where a darker contrast line appears in the middle of
the nanowire due to the largest thickness in the diagonal direction
of the nanowire of a square cross section. A schematic summary
of the nanowire morphology is illustrated in Figure 2c: both the
tip of the nanowire and the side surfaces are terminated with the
{100} crystallographic lattice planes as labeled in the illustration;
the dashed arrows represent the direction in which the electron beam
is incident with the projections labeled underneath using letters A
and B, which correspond to the images given in Figure 2a and
Figure 2b, respectively. After checking several other nanowires
among the same sample, we obtained similar results except that
some nanowires have cross sections that are rectangular rather than
square. The fact that catalyst particles were sometimes seen at the
end of the nanowires suggests that the formation of the CeB6
nanowires is likely a vapor-liquid-solid process.
In conclusion, we have developed a chemical vapor deposition
method that is able to produce successfully CeB6 nanowires of well-
defined morphology. The nanowires have a square cross section
with width of about 50 nm and have lengths extending more than
10 µm. The growth direction of the nanowires is the 001 lattice
direction, and they have flat tips and walls that are all terminated
by the {100} lattice planes. These CeB6 nanowires are of potential
use as point electron emitters for applications including providing
thermionic emission, field-induced emission, and thermal field-
induced emission of electrons for TEM, SEM, flat panel displays,
as well as other electronic devices that require high-performance
electron sources.
Figure 2. (a) [100] direction TEM image of the CeB6 nanowire tip. Inset
is a high-resolution lattice image of the tip’s top left corner. (b) [11h0]
direction TEM image of the same nanowire together with a high-resolution
lattice image showing its right-side wall. (c) Illustration of the 45° tilting
process to reconstruct the morphology of the nanowire.
Acknowledgment. We wish to thank the UNC Research
Council for financial support and J.T. is also partially supported
by Japan-U.S. Collaborative Scientific Research Program of JSPS.
TEM image at low magnification showing a straight nanowire with
smooth surfaces of lateral dimension around 50 nm and length
extending more than 10 µm. Parts b-d of Figure 1 show three
electron diffraction patterns taken along three different crystal zone
axes of the same nanowire to identify the CeB6 crystal lattice, which
has a primitive cubic structure of space group Pm3m and has lattice
constant a ) 0.4129 nm. The three zone axes are [100] (Figure
1b), [21h0] (Figure 1c), and [11h0] (Figure 1d). The relative
orientational relationship between the three electron diffraction
patterns are also given in the figures. For example, the [11h0] zone
axis electron diffraction pattern (Figure 1d) was obtained by tilting
the nanowire 45° about its [001] direction (illustrated in Figure
1b).
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Due to the important role of the emitter tip in the emission
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