APPLIED PHYSICS LETTERS 93, 153103 共2008兲

Mechanism and control of sidewall growth and catalyst diffusion on oxide
nanowire vapor-liquid-solid growth
Kazuki Nagashima, Takeshi Yanagida,a兲 Keisuke Oka, Hidekazu Tanaka, andTomoji Kawaib兲
Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047,
Japan

共Received 30 May 2008; accepted 17 August 2008; published online 13 October 2008兲
Oxide nanowires formed via the vapor-liquid-solid 共VLS兲 mechanism are attractive building blocks
toward nanowire-based electronic devices due to their fascinating physical properties. Althoughwell-defined oxide nanowires are strongly required for the applications, tapering during oxide
nanowire VLS growth has been detrimental and uncontrollable. Here we demonstrate themechanism to control the tapering during oxide VLS growth. Suppressing simultaneously both theoxidization of adatoms at the sidewall and the catalyst diffusion from the tip was found to beessential to avoid the tapering. This mechanism would be universal in various oxide nanowire VLSgrowths. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2978347兴Oxide nanowires formed by the vapor-liquid-solid
共VLS兲 growth mechanism and the heterostructures are natural candidates to incorporate various functionalities of transition metal oxides into nanowire-based devices.1–5 In developing these applications, it is important to fabricate long,straight nanowires with constant diameters.6–8 In many VLS
growths, tapering9–14 has been detrimental for the purpose
and sometimes uncontrollable. However, a comprehensive
understanding of the key factors affecting the tapering is stilllacking due to the complex nature of oxide VLS growth.15,16We have therefore investigated the mechanisms and controlof the tapering in oxide VLS growth by carrying out MgO
nanowire VLS growth using pulsed laser deposition 共PLD兲
technique.17–23 MgO is a rocksalt crystal structure and has
been utilized as a versatile single crystal substrate for oxide
heterostructures due to the small lattice mismatch with various transition metal oxides.24,25 Since the MgO nanowires
can be a promising “nanosubstrate” for the nanowire
heterostructures,3–5,21,22 the well-defined nanowire morphologies are strongly desirable. Previously we have investigatedthe MgO VLS growth mechanisms and the kinetics.17–23
However, the control of the tapering has been unfeasible dueto the unknown physical mechanism. In this letter, we show
the mechanisms and controllability of the tapering in MgO
VLS growth. We found the significant role of the oxidation at
the sidewall in controlling the tapering phenomena. Thismechanism would be universal in various oxide VLS
growths.
MgO nanowires were grown on MgO 共100兲 single crystal substrate by Au catalyst-assisted PLD technique.17–23
Prior to the nanowire growth, Au catalysts were deposited on
MgO single crystal substrate. The background pressure of
the PLD chamber was 10−5 Pa. ArF excimer laser 共LamdaPhysik COMPex 102, ␭ = 193 nm兲 operating at the pulse
repetition rate of 10 Hz and the laser energy of 40 mJ wasused for the laser ablation. MgO single crystal was used forthe source of Mg species vapor during the nanowire growth.
Oxygen and argon mixed gas was introduced into the chama兲Electronic mail: yanagi32@sanken.osaka-u.ac.jp.
b兲Electronic mail: kawai32@sanken.osaka-u.ac.jp.
0003-6951/2008/93共15兲/153103/3/$23.00

ber with controlling the ambient pressure. The ambient pressure was 10 Pa with varying the ratio of oxygen and argon.
Prior to the laser ablation, the Au-coated MgO 共100兲 substrate was preheated at 800 ° C for 20 min. The growth temperature was 800 ° C. After the deposition, the samples wereannealed in oxygen atmosphere for 30 min and cooled downat room temperature for 30 min. The nanowire morphology
was characterized by field emission scanning electron microscopy 共FESEM兲 共JEOL JSM-6330FT兲 at an accelerating
voltage of 30 kV. High-resolution transmission electron microscopy 共HRTEM兲 共JEM-3000F兲 coupled with energy dispersive spectroscopy was used to evaluate the diameter, thecrystallinity, and the composition of the fabricated nanowires. The size of MgO nanowires was analyzed by averaging data for 500 samples in FESEM and for 300 samples in
HRTEM images for statistical reliability. HRTEM measurement was performed at an accelerating voltage of 300 kV.
Figure 1 shows the HRTEM and FESEM images of theMgO nanowires grown under pure oxygen pressure. It can beseen that the nanowires were grown over 3 ␮m for 300 min.
Note that such relatively long MgO nanowires are not feasible when reducing the ambient pressure due to the catalyst
diffusion from the tip, as reported elsewhere.20 The Au catalyst droplet remaining on the tip can be also seen in the[100]

[100]
500nm

100nm

FIG. 1. HRTEM image of MgO nanowire. The nanowire was grown under
10 Pa pure oxygen ambient pressure and 800 ° C growth temperature for300 min. The inset shows the FESEM image.
93, 153103-1© 2008 American Institute of Physics

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153103-2

Appl. Phys. Lett. 93, 153103 共2008兲

Nagashima et al.

(a)

500nm
FIG. 2. 共Color online兲 Schematic of oxide VLS growth with possible
mechanisms affecting the tapering phenomenon.

HRTEM image. More importantly it can be clearly seen thatthe nanowire diameter size tends to gradually increase below
the droplet. This indicates the occurrence of tapering during
the oxide nanowire growth even under relatively high ambient pressure, which cannot be interpreted in terms of previous implications.20 Since such tapering is highly detrimental
in controlling arbitrarily the nanowire morphologies, a way
to overcome this critical issue is strongly desired.
Considering the mechanisms of oxide VLS growth in
Fig. 2, there are several scenarios to explain the occurrenceof the tapering. The first most well known mechanism for thetapering phenomenon is based on the catalyst diffusion fromthe tip.9–12,20 Since the interface area between the liquid
droplet and the solid determines the nanowire diameter sizein principle, the change in the droplet size due to the catalyst
diffusion during growth results in the tapering. However, in
our experiments the maximum of the nanowire diameter appeared at the middle position of nanowires, as seen in theFESEM image. In fact this experimental trend is contradictive to the catalyst diffusion scenario since it is quite unlikely
to assume that the catalyst droplet size shows the maximum
during growth even in the presence of the catalyst diffusion.
In addition, our previous investigation20 clarified that thecatalyst diffusion from the tip at 10 Pa ambient pressure isnot significant compared with the VLS growth rate. Thus themechanism based on the catalyst diffusion from the tip alone
cannot explain the observed tapering phenomenon.
The other scenario is based on the sidewall growth dueto the crystallization at the sidewall. In this scenario, thecrystallization at the sidewall is not negligible comparedwith VLS growth rate. If this scenario was appropriate toexplain the observed tapering phenomenon, reducing thecrystallization at the sidewall would be crucial in preventing
the detrimental tapering. Considering the mechanisms of thecrystallization at the sidewall, there are two possible factors
to suppress the sidewall growth, such as 共i兲 enhancing thediffusion length of adatoms with the incorporation of adatoms into the catalyst droplet and 共ii兲 reducing the oxidation
at the sidewall. Thermodynamically reducing the ambient atmosphere, i.e., increasing the temperature and reducing theoxygen pressure, satisfies both requirements. However, increasing further the ambient temperature from the current
temperature of 800 ° C might cause the evaporation of Au
catalyst droplet,18 and in our experimental setup the ambienttemperature range above 900 ° C is not feasible. On the otherhand, it is quite controllable to reduce the oxygen pressure.
However, reducing the ambient oxygen pressure enhances
the catalyst diffusion from the tip, resulting in the tapering,
as reported elsewhere.20 Thus here we have a dilemma toreduce the sidewall growth via controlling the ambient oxy-(b)

500nm

(c)

500nm
FIG. 3. FESEM images of nanowires when varying the mixed gas ratio.

gen pressure. The way to overcome simultaneously both thesidewall growth and the catalyst diffusion issues would be touse a mixed ambient gas of oxygen and argon with varying
the oxygen partial pressure. The use of the mixed gas isbased on the following reasons. First, controlling the total
ambient pressure enables to suppress the catalyst diffusionfrom the tip. Second, reducing the oxygen partial pressure
under constant total ambient pressure suppresses the oxidation at the sidewall without the catalyst diffusion from thetip.20 Thus, this method is promising to control simultaneously the sidewall growth and the catalyst diffusion issues
in principle.
Figures 3共a兲–3共c兲 show the FESEM images of the MgOnanowires grown when varying the mixed gas ratio of oxygen and argon. The drastic variation in the nanowire morphology can be seen in the images, indicating the significant
role of the mixed gas ratio on the oxide VLS growth. Figure
4 shows the dependence of the mixed gas ratio on the degreeof the tapering and the nanowire diameter size. Here we
define the degree of the tapering Td as Td = 共Db − Dt兲 / Db
⫻ 100, where Db is the diameter at 500 nm below from thetip and Dt is the diameter at the tip. The diameters were
measured from HRTEM images for 300 samples. The nanowire diameter size was measured near the tip. The dataclearly demonstrate that the tapering nature is strongly suppressed when reducing the oxygen partial pressure underconstant total ambient pressure. In addition the use of relatively high total ambient pressure retained the catalyst droplet on the tip via suppressing the catalyst diffusion, resulting
in a nonsignificant dependence on the diameter size near thetip. Figure 5 shows the HRTEM images and selected area
electron diffraction 共SAED兲 pattern of nanowires grown under 1:1000 of O2 : Ar ratio. The nanowire morphology clearly

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153103-3

Appl. Phys. Lett. 93, 153103 共2008兲

Nagashima et al.

In summary, we have demonstrated the mechanism tocontrol the tapering during oxide VLS growth, which had
been uncontrollable. Suppressing both the oxidization ofadatoms at the sidewall and the catalyst diffusion from thetip was found to be essential to avoid the tapering. Thismechanism would be universal in many oxide nanowire VLSgrowths.
1FIG. 4. 共Color online兲 Variation in the degree of tapering 共dotted line兲 andthe tip diameter 共solid line兲 when varying the mixed gas ratio.

differs from that in Fig. 1 with pure oxygen ambient atmosphere. The untapered shape for the long scale and the highcrystallinity were confirmed. The observed suppression ofthe tapering when reducing the oxygen partial pressure indicates that the oxidation at the sidewall plays an importantrole on the sidewall growth. Thus the above experimental
results highlight that it is possible to control simultaneously
the sidewall growth, the catalyst diffusion, and the resultanttapering, which have been very detrimental and uncontrollable. Since oxide nanowires must be fabricated under oxygen ambient atmosphere, the mechanism proposed hereinwould be universal in many oxide VLS growths.
(200)
(020)

[100]

100nm

FIG. 5. HRTEM image of MgO nanowire. The nanowire was grown under
10 Pa total ambient pressure with 1:1000 of O2 : Ar and 800 ° C growthtemperature for 300 min. The inset shows the SAED pattern.

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