Field-emission induced growth of nanowires

Article abstract of DOI:10.1063/1.1529084

Field-emission induced growth of nanowires was discussed. The single nanowires were grown in the prescence of an organometallic precursor. It was found that it is possible to grow sub-20-nm nanowire system which depends on the choice of suitable precursor

Full text of DOI:10.1063/1.1529084

Field-emission induced growth of nanowires
J. T. L. Thong, C. H. Oon, M. Yeadon, and W. D. Zhang
Citation: Applied Physics Letters 81, 4823 (2002); doi: 10.1063/1.1529084
View online: http://dx.doi.org/10.1063/1.1529084
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 25
16 DECEMBER 2002
Field-emission induced growth of nanowires
J. T. L. Thonga) and C. H. Oon
Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering
Drive 3, Singapore 117576, Singapore
M. Yeadon
Department of Materials Science, National University of Singapore, 10 Science Drive 4, Singapore 117543
and Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore
W. D. Zhang
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore
͑Received 21 May 2002; accepted 16 October 2002͒
Nanowires are grown from a cold-field-emission tip in the presence of a precursor, typically an
organometallic or organic compound. Electron emission from the newly grown nanowire tip
continues the growth and can give rise to nanowires that are tens of microns long. Single nanowires
are obtained by limiting the field-emission current to values of typically 100 nA or less. Tungsten
nanowires with diameters of less than 4 nm have been grown from W(CO)6 . Other nanowires
grown include cobalt, iron, and carbon. Composite wires can be fabricated by continued growth
with different precursors. Nanowires have been grown on etched wire tips, carbon nanotubes and
scanning probe tips. Voltages applied to electrodes on an integrated circuit die can be used to attract
a nanowire towards and contact a biased electrode. By such means, it is possible to connect the end
of a pointed structure, such as a carbon nanotube, to an electrode. © 2002 American Institute of
Physics. ͓DOI: 10.1063/1.1529084͔
Nanowires and tubes have attracted research interest in
recent years as their functional properties hold promise for
incorporation into nanoscale devices and systems. In particu-
lar, carbon nanotubes have been used as field-emission
sources,1 scanning probe microscope tips,2,3 and nanoscale
electronic devices.4–7 Although carbon nanotubes can be se-
lectively grown by catalytic growth on prepatterned catalyst
films8–10 or other templates,11 the nanotubes are often grown
ex situ, harvested, and redeposited, micromanipulated, and
connected as required for electrical characterization. These
procedures are followed for other types of metallic and semi-
conductor nanowires grown by electrodeposition12 and vari-
ous schemes based on the vapor-liquid-solid ͑VLS͒
mechanism.13–15
The growth of metallic dendritic structures had previ-
ously been observed to result from electron emission from a
cathode in the presence of metal carbonyls. The technique
has been used for the production of field-ion emitters.16
Similar dendritic structures were obtained by thermal and
cold-field emission using Cr(CO)617 and Mo(CO)6 .18 It was
proposed that the needles grow by metal ions supplied from
a corona plasma that is sustained by the field-emitted elec-
trons.
of which forms the field emission source. Single-nanowire
growth is achieved by controlled low-current emission. In
contrast to ex situ growth techniques, the present technique
allows the growth of a nanowire at a defined point that is
simply a sharp tip. The point of initiation naturally provides
a mechanically strong electrical anchor for the nanowire,
while the other end of the nanowire can be contacted to
another electrode by electrostatic attraction to complete a
circuit where desired.
We carried out nanowire growth in the chamber of an
environmental scanning electron microscope to facilitate im-
mediate viewing during and after growth. The precursor gas
is introduced into the chamber through a nozzle directed at
the growth site around 1 mm away. In our setup, the nozzle
also acts as an anode that is biased typically at a few hundred
volts relative to the pointed tip. A feedback control to the
high-voltage supply allows the field-emission system to op-
erate in constant current mode which is important to achieve
sustained single-nanowire growth. Prior to the admission of
the precursor, relatively high voltages, at an anode–cathode
spacing of typically 100 ␮m, are required to obtain a field-
emission current of several tens of nA from a tip of around
50 nm radius. Upon admission of the gas, the voltage re-
quired to maintain the same current drops rapidly, after an
initiation period, to typically 80–200 V as the result of the
growth of a nanowire with much smaller tip radius. Figure 1
shows the rapid decrease in the anode voltage, initially at 3
kV, upon admission of the precursor gas into the chamber at
a time indicated by point A. At point B, the current compli-
ance of the control loop is reached, and the system subse-
quently operates in a constant-current mode. This period of
decreasing voltage corresponds to the growth of a material
cone at the point of field emission. The voltage profile then
Here we report on the growth of single nanowires by
cold-field emission in the ambient of a suitable precursor
gas, typically an organometallic compound. Nanowire
growth is initiated at a sharp tip where a sufficiently high
electric field causes field emission to occur. The electrons
dissociate precursor molecules ahead of the tip. Ionized mo-
lecular fragments attracted back towards the tip result in the
self-sustained growth of a conductive nanowire, the tip front
a͒
Electronic mail: elettl@nus.edu.sg
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4824
Appl. Phys. Lett., Vol. 81, No. 25, 16 December 2002
Thong et al.
FIG. 2. ͑a͒ TEM ͑300 keV, Philips CM300͒ micrograph of two relatively
thick tungsten tips grown at high current, followed by a reduction in the
growth current to produce smaller nanowire branches. Point X shows the
initiation of a new nanowire at the shank of the right-hand side tip. Inset
shows the electron diffraction pattern near the tip of the shank confirming
that the material is tungsten. ͑b͒ Higher magnification images of the smaller
nanowires show that the tungsten core has a diameter of less than 4 nm
towards the leading end. The amorphous carbonaceous coating which pro-
gressively thins out towards the growing tip.
FIG. 1. Anode voltage during nanowire growth. Precursor gas admitted at
A. The transition at B is due to the control loop kicking in to operate in
constant current mode.
flattens out in the steady-state nanowire growth regime.
Nanowire growth is easily carried out on naturally sharp
tips such as electrochemically etched tungsten tips or atomic
force microscope ͑AFM͒ tips, the latter providing an easy
self-aligned route to fabricating fine, high-aspect ratio tips on
a conventional scanning probe tip.19 Using tungsten hexa-
posed growth mechanism is the dissociation and ionization
of the precursor molecule by the narrow cone of electrons
emerging from the field emitter: this is in contrast to growth
arising from nonspecific electron emission results in massive
dendritic structures.22 As the threshold electron energies re-
quired to form Wϩ and W(CO)xϩ ions from W(CO)6 lie
between 8 and 23 eV,23 the peak ion distribution is at a short
distance ahead of the tip as a result of the diverging electron
beam and decrease in ionization cross-section at larger elec-
tron energies. Fragmentation of the molecule imparts a radial
velocity component depending on the molecular species in-
volved which means that the ionized fragments may not nec-
essarily follow the electron trajectories back to the electron
emission point on axis. Ions formed further away, and those
with substantial radial velocities would impinge on the nano-
wire shaft causing it to thicken. A steady-state deposition
profile is established whereby the electron-emission cone
from the formed tip ionizing the molecules and the ensuing
ion trajectories deposit a new tip surface that is identical to
the underlying tip profile. Field-induced reactions at the tip
may be responsible for further molecular dissociation at the
growth front.
Positive ions attracted back towards the field emission
tip are well known to be responsible for the sputtering of
field emitters.24 However, in the present case, the molecular
flux involved is much higher, with gas pressure at the deliv-
ery nozzle estimated at 1 mbar, and the ionized species im-
pinging on the cathode causes material buildup. At lower gas
pressures, nanowire growth becomes erratic, which is be-
lieved to be due to a competing process of sputtering by
high-energy ions. Composite nanowires can be grown by in-
terrupting the growth and switching the precursor. In this
manner, we have grown tungsten/cobalt/tungsten, tungsten/
iron and other such systems.
carbonyl W(CO) as the precursor, tungsten nanowires
͓
͔
6
with core diameters as small as 3–4 nm and up to several
microns in length can be grown. Low field-emission currents
of around 100 nA are needed to maintain single-nanowire
growth. Under similar growth conditions, iron nanowires
grown from iron pentacarbonyl Fe(CO) are comparable
͓
͔
5
in diameter, typically less than 10–15 nm. On the other hand,
cobalt nanowires grown from dicobalt octacarbonyl
Co (CO) and cobalt tricarbonyl nitrosyl Co(CO) NO
͔
͓
͔
͓
2
8
3
are around 30 nm in diameter, while carbon nanowires
grown from acetylene are considerably thicker at around 250
nm. The diameter dependence on growth parameters and ma-
terial is currently being studied.
Higher currents provide higher growth rates and thicker
nanowires, but can give rise to random forking and the for-
mation of multiple nanowires. Figure 2͑a͒ shows a bright-
field TEM image of the tips of a typical treelike tungsten
structure. Point X on the micrograph shows the initiation of a
new nanowire. Electron diffraction patterns confirm that the
structure comprises tungsten polycrystals. Figure 2͑b͒ shows
that the leading edges of the tungsten nanowires are
ϳ3–4 nm in diameter. A buildup of amorphous carbon can
be observed along the length of the nanowires, becoming
gradually thinner closer to the growing tip. The deposit most
likely arises as a byproduct of the metal deposition process.
The carbonyl group released from the precursor during the
metal deposition reaction subsequently deposits along the
length of the growing nanowire. The carbon deposit may
serve the useful purpose of reducing the extent of oxidation
of the metal nanowires upon exposure to atmosphere.
Material deposition had previously been observed on the
tip in scanning-tunneling-microscopy ͑STM͒ assisted of
materials.20 Nanowire growth observed by Kent et al.21 on a
negatively biased STM tip was attributed to a field-induced
reaction. With the present technique, material growth at the
We demonstrated the use of metallic nanowires to con-
nect one end of a carbon nanotube ͑CNT͒. A sparse array of
70 nm diameter multiwalled CNTs was grown by plasma-
enhanced chemical vapor deposition ͑PECVD͒ using a mix-
ture of ammonia and acetylene and nickel catalyst on a sili-
con substrate. Field emission from a selected CNT in the
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tip is not observed in the absence of field-emission. The pro-
presence of tungsten carbonyl was initiated by bringing an
176.182.39.242 On: Tue, 06 May 2014 21:52:16

Appl. Phys. Lett., Vol. 81, No. 25, 16 December 2002
Thong et al.
4825
havior at room temperature. The measured resistance of 1
M⍀ is attributed largely to the series resistance of the
electron-beam induced deposited material. At much higher
currents, the tungsten nanowires were observed to deform
initially, and finally fused.
In summary, we have developed a technique for growing
single nanowires by a self-field-emission process in the pres-
ence of an organometallic precursor. As the nanowire growth
is initiated at a sharp point, the nanowires are well-anchored
both electrically and mechanically to this point to form free-
standing nanowires that can be used for scanning probe tips,
and obviously for field-emission tips. Longer nanowires can
be used for interconnection to the ends of nanostructures
such as carbon nanotubes. We have also grown composite
nanowires by switching precursors and continuing the
growth. With the choice of suitable precursors to ensure
compatibility between the diameters of the grown nanowires,
it is possible to grow sub-20-nm nanowire systems such as
an iron/tungsten segment on the end of a carbon nanotube.
The deposition of composite magnetic nanowires would
open up the possibilities of exploring spin electronics in such
structures.
FIG. 3. ͑a͒ Tungsten nanowire grown on carbon nanotube. The discontinuity
indicated by the arrow is due to an interruption in the growth for an inter-
mediate viewing of the nanowire which was bent, but still free-standing at
this point. The final length of the nanowire allowed it to contact the silicon
substrate after the removal of the anode bias. Inset: higher-magnification
micrograph showing the attachment of the nanowire to the tip of the carbon
nanotube. The estimated diameter of the nanowire is less than 15 nm upon
initial viewing in the SEM, but appears thicker in the micrograph due to
contamination buildup; ͑b͒ a forked pair of tungsten nanowires grown by
field emission from cone-like structure deposited via electron beam deposi-
tion from tungsten carbonyl.
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