Single-crystalline LaB6 nanowires

Article abstract of DOI:10.1021/ja043512c

Lanthanum hexaboride nanowires produced by chemical vapor deposition are single-crystalline and grown along the [111] direction. Streaks appearing in the electron diffraction spots indicate the lateral direction perpendicular to the nanowire growth axis.

Full text of DOI:10.1021/ja043512c

Published on Web 02/11/2005
6
Single-Crystalline LaB 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 October 26, 2004; E-mail: lcqin@physics.unc.edu
Lanthanum hexaboride (LaB
6
) is widely used in modern technol-
high compared to that of LaB
scientific and practical significance to fabricate and investigate
nanoscale LaB , the compound being considered as the best electron
emitter material thus far.
We herein report the synthesis and structural characterization of
LaB nanowires with diameter smaller than 100 nm and length more
6
than a few tens of micrometers. We have developed and applied a
chemical vapor deposition (CVD) method that produces single-
6
. Therefore, it is of both great
ogy as an excellent thermionic electron emission source which offers
high brightness and long service life. The advantages are originated
from its low work function and nature of low volatility.1 Extensive
thermal field-emission tests have also been performed with LaB
These studies revealed that, compared to the LaB thermionic
electron emitters, a brightness of at least 2 orders of magnitude
higher could be achieved with the LaB thermal field emitter. The
6
,2
3-5
6
.
6
6
required local electric field is only half of that needed by the typical
commercial thermal field emitters made with zirconiated tungsten
sharp tips.
6
crystalline LaB nanowires of uniform diameter and well-defined
growth direction. The synthesis is based on the following chemical
reaction:1
5-17
6
For a LaB thermionic electron emitter, a smaller emitter tip
2
LaCl (g) + 12BCl (g) + 21H (g) ) 2LaB (s) + 42HCl(g)
3 3 2 6
diameter results in a brighter electron beam and a smaller minimum
electron beam spot size, which will in turn improve greatly the
resolution and performance of electron optical instruments such as
the transmission electron microscope (TEM) and the scanning
The reaction was conducted in a tube furnace operated at 1150 °C
where the BCl gas was introduced to the reaction zone in a stainless
steel tube and LaCl powders were vaporized in a tantalum bowl
placed inside the tube furnace. The reaction was conducted in an
atmosphere at 1 atm pressure of hydrogen and nitrogen (5%H
) gases. Single-crystalline nanowires were grown on gold metal
3
3
electron microscope (SEM) that are often equipped with a thermi-
onic LaB
commercial LaB
6
electron emission cathode.6 The tip diameter of
-8
2
+
6
thermionic electron guns is typically in the range
N
2
of ∼5-100 µm. The emitter tip size plays an even more important
role in the thermal field-emission applications such as the Shottky
electron gun, which at present is considered as the best high-
resolution electron filament. With a low work function as one factor,
a strong local electric field is the other factor to determine the
electric field-induced emission for a given applied voltage and
cathode gun geometry. For a rod-shaped field-induced electron
emitter, the local electric field F which extracts electrons from the
emitter tip can be expressed as a function of the applied voltage V,
the emitter tip radius r, its longitudinal length h, and the distance
d between the anode and the substrate where the emitter stands: 9
films coated on silicon substrates. Single-crystalline whiskers can
also be produced under similar conditions.
For electron diffraction and microscopy observations, the LaB
6
nanowires were first dispersed in ethanol and then collected onto
a copper grid that was coated with a thin holey carbon film. The
TEM (JEM-2010F equipped with a Shottky field-emission gun)
was operated at 200 kV during structural characterization.
A representative morphological study of the nanowires is
presented in Figure 1. The nanowires with smooth surfaces of
diameter around 100 nm and length extending to more than a few
tens of micrometers were observed and measured using both
electron imaging and selected-area electron diffraction. Figure 1a
is a typical TEM image at low magnification where the dimensional
morphology is displayed. Figures 1b-d show three electron
diffraction patterns taken from three different crystal zone axes of
0.9
F ) 1.2V(2.5 + h/r) [1 + 0.013d/(d - h) -
0.033(d - h)/d]/d
The above relationship indicates that the smaller the emitter tip
radius r and the higher its aspect ratio h/r, the greater the local
electric field F will become, which in turn will enable field-induced
electron emission to occur at a lower applied voltage compared to
emitters of the same material but of larger dimensions. This lowered
turn-on voltage for field-induced electron emission has been
6
the same nanowire to identify the LaB crystal lattice, which has a
primitive cubic structure of space group Pm3m and has lattice
constant a ) 0.4153 nm. The three zone axes are [0 1 -1] (Figure
1b), [-1 2 -1] (Figure 1c), and [-1 4 -2] (Figure 1d). The relative
orientational relationship between the three electron diffraction
patterns is also given in the figures. For example, the [-1 2 -1]
zone axis electron diffraction pattern (Figure 1c) was obtained by
rotating the nanowire 30° about its [111] direction (illustrated in
Figure 1b). It can also be seen from the diffraction patterns that all
reflection spots are elongated to become streaks and these streaks
are always perpendicular to the [111] lattice direction, indicating
observed in many nanoscale structures including carbon nanotubes
and nanowires of SiC, W, CuS, and MoO
3
materials.1
0-14
The small
diameter and large aspect ratio of such one-dimensional nanostruc-
tures magnify enormously the local electric field at the emitter’s
free end and give greatly enhanced electric field-induced emission
from these materials whose work functions are even considered
that the nanowire axis is parallel to the [111] direction of the LaB
6
crystal. This result was further confirmed by high-resolution
transmission electron microscopy (HRTEM).
HRTEM was applied to analyze the detailed atomic structure
around the nanowire tip. Figure 2a is an electron microscope image
†
Curriculum in Applied and Materials Sciences, University of North Carolina
at Chapel Hill.
‡
Department of Physics and Astronomy, University of North Carolina at Chapel
Hill.
§
National Institute for Materials Science.
2862
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J. AM. CHEM. SOC. 2005, 127, 2862-2863
10.1021/ja043512c CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
faceted lattice planes are displayed in Figures 2b-g in correspond-
ing sequence. Figure 2b shows the lattice image of the stem area
(marked B in Figure 2a) of the LaB
6
nanowire. The axial direction,
[111], which is the same of the growth direction, of the nanowire
has also been indicated in the figure. Figure 2c shows the lattice
image near facet C of Figure 2a, whose surface is terminated with
the (1 1 -1) lattice plane. Figure 2d shows the area marked with
letter D in Figure 2a, which has a (110) facet. Figure 2e shows a
(111) facet, which is equivalent to Figure 2c in atomic structure
with the same surface energy and is perpendicular to the nanowire
axis [111]. Figure 2f shows (211) facet corresponding to the area
marked with F in Figure 2a. It is interesting to note that facet F is
the smallest compared with others shown in Figure 2a. Area G,
shown in Figure 2g, is a facet of (100) which is a low index lattice
plane. By examining the areas covered by the various facets
characterized, we found that the (100), (110), and (111) planes are
the dominant terminating facets. Lattice planes of higher indices
appeared only as transitions between the above dominant facet
planes of low indices. This result agrees with the lowest surface
energy principle in crystal growth.
6
Though commercial LaB electron gun filaments are usually
made along the 〈100〉 direction of the crystal lattice due to the {100}
plane’s lower work function and higher symmetry, it has been
Figure 1. (a) Low-magnification TEM image of a LaB6 nanowire. (b-d)
1
Electron diffraction patterns of the same LaB6 nanowire tilted to zone axes
suggested that the 〈111〉 oriented LaB
be an even better alternative since they offer better stability.
Electric field-induced electron emission measurements on the LaB
6
single-crystalline tips might
[0 1 -1], [-1 2 -1], and [-1 4 -2], respectively. Dash lines indicate the
1
8
tilting axes about which the nanowire was rotated to obtain the needed
orientation. Circular arrows give the tilting angles, and the straight arrows
point to the tilting sequence.
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nanowires are in progress.
In conclusion, we have developed a CVD method that has been
able to produce successfully LaB nanowires of well-characterized
6
morphology. The nanowires have a diameter around and smaller
than 100 nm and length extending to a few tens of micrometers.
The growth direction of the nanowires is its 〈111〉 lattice direction,
and they have hemispherical tips that are terminated by lattice planes
of low indices such as {100}, {110}, and {111}. The LaB
6
nanowires’ potential applications include providing thermionic
emission, field-induced emission, and thermal field-induced emis-
sion of electrons for TEM, SEM, flat panel displays, as well as
other electronic devices that require high-performance electron
sources.
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Figure 2. (a) High-resolution image of the LaB6 nanowire tip with
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9
of the nanowire tip. The tip is hemispherical, and it is terminated
by several different lattice planes that are labeled with letters B
through G on the image. The HRTEM images characterizing these
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