Published on Web 09/03/2005
Single-Crystalline GdB6 Nanowire Field Emitters
Han Zhang,† Qi Zhang,‡ Gongpu Zhao,‡ Jie Tang,‡,§ Otto Zhou,†,‡ and Lu-Chang Qin*,†,‡
Curriculum in Applied and Materials Sciences, 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 June 28, 2005; E-mail: lcqin@physics.unc.edu
Rare-earth hexaborides are the best thermionic electron sources
due to their low work function, low volatility at high temperature,
high conductivity, high chemical resistance, and high mechanical
strength.1 Single-crystalline LaB6 and CeB6 (with work functions
of about 2.5 eV) have shown this advantage in thermionic electron
emission applications during the past 50 years. In the rare-earth
hexaboride family, GdB6 is believed to have the lowest work
function (∼1.5 eV).2 However, satisfactory thermionic emission
has not been achieved with this material, likely due to the relatively
poor stability of GdB6 at high working temperature around 1500
°C.2-4 Nevertheless, GdB6’s extremely low work function offers a
great opportunity for making this material room temperature field
emitters that put less stringency on the high-temperature stability.
A field emission electron source offers a brightness more than 100
times higher than the conventional thermionic electron sources, and
its current density J can be expressed by the Fowler-Nordheim
equation (in SI units):5,6
(CVD) system described before,7 except that GdCl3 powders
(99.99%, Aldrich) were used as the evaporation source and a clean
silicon wafer was used as the deposition substrate.
After the reaction, a scanning electron microscope (SEM, JEM-
6300) equipped with an energy-dispersive X-ray spectrometer
(EDX, KevexSigma3) was used to examine the nanowires. The
lateral dimensions of the nanowires range from below 100 nm to
more than 1 µm. EDX analysis indicates the nanowires are
composed of B and Gd elements.
The GdB6 nanowires were also examined in a transmission
electron microscope (TEM, JEM-2010F) operated at 200 kV. Figure
1a is a morphological image of the grown GdB6 nanowires. They
are typically 50-60 nm in lateral dimensions and are more than
several microns in length. The nanowires’ tip-top surfaces are flat
and form a right angle with the side surfaces, as can be clearly
seen in the inserted high-resolution image of the area marked by
an arrow. The nanowires were found to grow along their 001
lattice directions, and both the tip-top and the side surfaces are
terminated with the {100} lattice planes. Panels b and c of Figure
1 are the TEM images together with their corresponding selected
area electron diffraction patterns taken from a single GdB6 nanowire
along its [100] and [21h0] lattice directions, respectively. A simple
geometric calculation was performed, as illustrated in Figure 1d,
in order to obtain the morphology of the nanowire tip. By using B
) 56 nm measured from Figure 1b, C ) 68 nm from Figure 1c,
and θ ) 26.6° determined from the two diffraction patterns, we
obtained the nanowire’s other side length A from the equation A )
(C - B cos θ)/sin θ ) 40 nm.
To study its field emission properties, a GdB6 nanowire with
lateral dimension of about 200 nm was picked up under an optical
microscope assisted with a manipulator, and the nanowire was then
attached onto an etched 0.5 mm tungsten (W) tip with an acrylic
adhesive. Field emission was measured in a high vacuum chamber
of 10-7 Torr. With the W tip as the cathode and a phosphor-coated
ITO glass as the anode spaced at 250 µm, an increasing electric
voltage was then applied and, in the meantime, the emission current
was measured and the emission patterns were recorded with a CCD
camera simultaneously.
Figure 2a shows the I-V curve of the field emission from the
GdB6 single nanowire. An emission current of 10 nA was obtained
at a voltage of 650 V, and the emission current reached 200 nA
before the emitter broke down. During the entire emission process,
the emitter’s surface is estimated to reach a maximum temperature
of about 400 °C, which is much lower than the melting point (2510
°C) of GdB6, but is much higher than the acrylic adhesive’s
tolerance temperature of 60 °C. The large current fluctuations
observed in the I-V curve are therefore suggested to be caused by
the melting of the adhesive attaching the GdB6 nanowire to the W
tip. A Fowler-Nordheim plot (ln(I/V2) vs 1/V) is depicted in Figure
2b, revealing characteristics of metallic field emission. Figure 2c
is a low-magnification TEM image of the GdB6 nanowire emitter,
J ) 2.23 × 10-25(E2/φ) exp(4.12 × 10-9/φ1/2 - 1.02 ×
1038 φ3/2/E) A/m2
where E is the local 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 increases rapidly with an increasing
electric field E, which can be expressed as E ) âV, with V being
the voltage applied on the emitter tip and â a geometry-dependent
enhancement factor that becomes larger as the emitter tip is sharper.
It can also be seen that the emission current density J is enhanced
almost exponentially with the decrease of work function, φ. A recent
study also indicates that, in electron optical applications, reducing
the emitter’s work function is the only way to achieve high
brightness without increasing the electron energy spread, which
causes chromatic aberrations.6 Therefore, to obtain a high emission
current with small energy spread at a convenient working voltage,
sharp field emitters out of low work function materials, for example,
rare-earth hexaboride nanowires, are desired. In our previous work,
we have demonstrated the synthesis of 111 oriented LaB6
nanowires, 001 oriented LaB6 nanowires, and 001 oriented CeB6
nanowires and also demonstrated field emission from a single LaB6
nanowire emitter with a current density as high as 5 × 105 A/cm2
at a working voltage of 800 V.7-9 In this communication, we present
for the first time a successful synthesis of single-crystalline GdB6
nanowires and a measurement of their field electron emission.
The synthesis is based on the following chemical reaction:
2GdCl3(g) + 12BCl3(g) + 21H2(g) ) 2GdB6(s) + 42HCl(g)
The reaction was conducted in a similar chemical vapor deposition
† Curriculum in Applied and Materials Sciences, University of North Carolina.
‡ Department of Physics and Astronomy, University of North Carolina.
§ National Institute for Materials Science.
9
13120
J. AM. CHEM. SOC. 2005, 127, 13120-13121
10.1021/ja054251p CCC: $30.25 © 2005 American Chemical Society
Zhang, Han
