APPLIED PHYSICS LETTERS 90, 233103 共2007兲

Control of magnesium oxide nanowire morphologies by ambienttemperature
Kazuki Nagashima, Takeshi Yanagida,a兲 Hidekazu Tanaka, and Tomoji Kawai
Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki,
Osaka 567-0047, Japan

共Received 26 March 2007; accepted 13 May 2007; published online 4 June 2007兲
Although controlling morphologies of oxide nanowires formed using vapor-liquid-solid 共VLS兲
mechanism is desired in developing functional oxide-nanowire applications, a comprehensive
understanding of the key factors affecting oxide-nanowire VLS growth is still lacking. Here, theauthors demonstrate the controllability of magnesium oxide nanowire morphologies by varying theambient temperature and discuss the underlying mechanism. Decreasing the ambient temperature
resulted in shorter, tapered, and square-rod shaped nanowires, whereas increasing the ambient
temperature allowed fabricating longer and untapered nanowires. The variation of oxide-nanowire
morphologies is interpreted in terms of the competition between the VLS growth and the sidewall
growth due to the variation of adatom transport. © 2007 American Institute of Physics.
关DOI: 10.1063/1.2746086兴
One-dimensional inorganic nanostructures formed using
vapor-liquid-solid 共VLS兲 growth mechanism are potential
candidates for a range of device applications in nanoelectronics, optoelectronics, and biosensors.1–3 Although, the investigations in this field have been concerned with silicon andcompound semiconductors,4–8 integrating functionalities ofvarious materials into the nanowires might expand the application range of nanowire-based devices. For example, oxidematerials are known to exhibit a rich variety of physicalproperties including ferromagnetism, ferroelectric, andsuperconducting,9–11 and it would be an interesting and challenging issue to incorporate such diverse functionalities ofoxide materials into nanowires. In developing such functional oxide nanowires for applications, controlling arbitrarily morphologies of oxide nanowires formed using VLS
is strongly desired. However, a comprehensive understanding of the key factors affecting complex oxide nanowire VLS
growth is still lacking, since an understanding of the VLS
nanowire growth in Si nanowires, which has been most intensively investigated last decade, is even notcomprehensive.5–8 This has held back the arbitrary controllability of oxide nanowire morphologies. Magnesium oxide
共MgO兲 has been used as a versatile single crystal substrate
for oxide thin film growth due to the small lattice mismatchwith various functional transition metal oxides.9–11 As suchthe feasibility and controllability of MgO nanowires enable
us to integrate arbitrarily desired transition metal oxides into
the oxide nanowires via heteronanowires.12,13 There are several experimental parameters to control the nanowire morphologies in VLS growth, including catalyst droplet size,pressure, temperature, and so on.6,14,15 The variation of theambient temperature strongly affects the VLS growth andconsequently the nanowire morphologies.15 However, therole of such temperature variation on the oxide VLS growth
is not fully understood.16,17 Here, we demonstrate the crucialrole of the ambient temperature to control the morphologiesof MgO nanowires formed using VLS growth. In addition,a兲Author to whom correspondence should be addressed; electronic mail:yanagi32@sanken.osaka-u.ac.jp

we discuss the underlying mechanism of the morphology
variation in terms of the transport of adatoms during VLS
growth when varying the ambient temperature.
MgO nanowires were grown on MgO 共100兲 single crystal substrate by Au catalyst-assisted pulsed laser deposition
technique. Prior to the nanowire growth, Au catalysts weresputtered with the thickness of 1 nm. ArF excimer laser 共␭= 193 nm兲 operating at the pulse repetition rate of 10 Hz andthe laser energy of 40 mJ was used for the laser ablation.
MgO single crystal was used as a target. The ambient oxygenpressure was kept to be 10 Pa during depositions. The substrate temperature for growth was varied from500 to 850 ° C. The nanowire morphology was characterizedby field emission scanning electron microscopy 共FESEM兲共JEOL JSM-6330FT兲. The length and the size of MgO nanowires were analyzed by averaging data for 500 samples in
FESEM images for statistical reliability. High-resolution
transmission electron microscopy 共HRTEM兲 共JEM-3000F兲
coupled with energy dispersive spectroscopy 共EDS兲 wasused to evaluate the diameter, the crystallinity, and the composition of the fabricated nanowires. The tip size of MgOnanowires was analyzed by averaging data for 300 samples
in HRTEM images. Atomic force microscopy 共AFM兲 共Veeco
DI3100 tapping mode兲 was used to measure the morphologyof Au catalyst droplet before nanowire growth.
Figure 1 shows the FESEM images of the MgO nanowires grown on MgO 共100兲 single crystal substrate whenvarying the ambient temperature. The nanowires were grownperpendicular to the substrate, indicating the epitaxial growthof the MgO nanowires. Figures 1共a兲 and 1共b兲 show the topview images of the MgO nanowires grown at and 500
800 ° C, respectively. The variation of nanowire morphologies in the side-view images when varying the ambient temperature was shown in the middle between Figs. 1共a兲 and1共b兲. The morphologies of nanowires tend to be shorter, tapered, and square-rod shape as decreasing the ambient temperature. In addition, in the top-view image the diameter size
tends to increase as decreasing the ambient temperature. On
the other hand, increasing the ambient temperature results inlonger and relatively untapered nanowires.

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Nagashima et al.

FIG. 1. 共Color online兲 FESEM images of MgO nanowires grown on MgO共100兲 single crystal substrate when varying the growth temperature. 共a兲 The
top-view image of nanowires grown at 500 ° C and 共b兲 800 ° C. The FESEM
images in the middle between 共a兲 and 共b兲 show the side-view structures. The
schematic illustration of observed trend is shown above the side-view
images.

Figure 2 shows the ambient temperature dependence on
the nanowire growth rate in the length. The growth rate systematically increased with increasing the ambient temperature below 750 ° C, and through the maximum then decreased. There are two possible factors affecting the MgOnanowire growth when varying the ambient temperature,such as 共i兲 the diffusion of adatoms and 共ii兲 the desorption
and evaporation of adatoms.18 It is well known that the diffusion of adatoms is enhanced with increasing the ambient
temperature.18 Thus the increase of ambient temperature induces faster diffusion on the surface and also increases theequilibrium concentration of adatoms. These result in theincrease of the diffusion flux to the catalyst at the nanowire
tip, and consequently the enhancement of nanowire growth
below 750 ° C observed in Fig. 2. On the other hand, increasing the ambient temperature gradually enhances the desorption and evaporation of adatoms from the substrate surface
and the sidewall.18 This effect essentially limits the adatom
flux to the nanowire tip. Therefore, the competition between
the diffusion and the desorption of adatoms when varying theambient temperature plays an important role on the nanowire

Appl. Phys. Lett. 90, 233103 共2007兲

FIG. 3. 共Color online兲 HRTEM images of MgO nanowires when varying theambient temperature: 共a兲 500 and 共b兲 800 ° C. Framed area highlights thedifference between high temperature and low temperature grown MgO
nanowires in the morphology near the tip.

growth via changing the adatoms flux to the catalyst at thetip.18
Figure 2 also shows the ambient temperature dependence on the diameter sizes of nanowires at both the bottom
and the tip near the catalyst. The tip diameters were measured by both TEM and SEM images, and the bottom diameters were identified from SEM images. In relatively low
temperature range below 700 ° C, the size difference between
the bottom and the tip diameters was found, highlighting theoccurrence of tapering. Such tapering tends to be more significant as the ambient temperature decreases. In higher ambient temperature above 800 ° C, the decrease of both thebottom and the tip was observed. In fact, similar trend whenincreasing the growth temperature was recently reported onZnO nanowire growth.19 There are two possible scenarios tointerpret the occurrence of tapering. One is based on thecatalyst diffusion from the tip during growth.5,8 Since thenanowire diameter size is determined by the catalyst size
within the framework of the VLS mechanism, such catalyst
diffusion causes the tapering via changing the catalyst size
during growth.14 Generally, increasing the ambient temperature enhances the diffusion of catalyst, and consequently thetapering in the VLS growth. This clearly contradicts with theexperimental trend, in which the tapering tends to occur withdecreasing the ambient temperature. The other scenario isbased on the effect of sidewall growth. This scenario assumes that 共i兲 the sidewall growth is not negligible comparedwith the VLS growth and 共ii兲 the adatom diffusion flux tends
to concentrate at the bottom position due to the limited diffusion length.
Figures 3共a兲 and 3共b兲 show the typical HRTEM imagesof the near-tip MgO nanowires fabricated at 共a兲 500 ° C and共b兲 800 ° C, respectively. Clear lattice fringes with the preferential orientation of MgO nanowire along 关100兴 direction
were observed in both images, indicating the high crystallinity. The eutectic formation of Au and Mg within catalyst
droplets seems to be responsible for VLS growth of MgO
nanowires, which is similar to GaAs nanowire formation,20
since HRTEM-EDS analysis for catalyst droplets demonstrated that the catalyst droplets were mainly composed ofAu with a small amount of Mg. The catalyst droplet size wasfound to be smaller for lower ambient temperature. If ashrinking of catalyst droplet via the diffusion during growthtends to occur as decreasing the ambient temperature, thismay cause the tapering, as assumed by the first scenario.
However, as mentioned above, the enhancement of catalyst

FIG. 2. 共Color online兲 Ambient temperature dependences on the MgO nanowire growth rate in the length and the diameter sizes of MgO nanowires atboth the bottom and the tip near the catalyst. The tip sizes 共red: by FESEM,
blue: by HRTEM兲 and the bottom sizes 共green: by FESEM兲. Although there
was a quantitative difference between FESEM and HRTEM values due tothe resolution and the metal coating for FESEM measurements, the variationin tip sizes was consistent in both measurements. All the nanowires weredeposited for 60 min.
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Appl. Phys. Lett. 90, 233103 共2007兲

Nagashima et al.

FIG. 4. 共Color online兲 Schematic illustration of the ambient temperaturedependent MgO nanowire growth. The
inset figures of 共a兲 and 共b兲 show theAFM images of the initial catalyst
droplet state before growth at 500 and800 ° C of the ambient temperature,respectively. 共c兲 and 共d兲 show theFESEM images of the MgO nanowire
morphologies after growth at 500 and
800 ° C of the ambient temperature,respectively. The scale bar is 50 nm
for AFM images and 100 nm forFESEM images.

diffusion with decreasing the ambient temperature is quite
unlikely. In addition, the size difference of catalyst droplets
was observed even in the initial catalyst droplets before
growth, as shown in the AFM images in Figs. 4共a兲 and 4共b兲.
The size ratio of catalyst droplets 共at the high temperature of
800 ° C: at the low temperature of 500 ° C兲 was found to bealmost 2 in both before and after the nanowire growth. Infact, such temperature dependence on catalyst droplet size
was reported elsewhere.21–23 These results highlight that thesize change of the catalyst droplet during growth is not significant in present experiments and at least not responsible
for the occurrence of tapering observed. Based on the second
scenario, the sidewall growth should tend to occur as decreasing the ambient temperature. In the nanowires grown athigh ambient temperature 800 ° C, the smooth sidewall andthe catalyst-size dependent nanowire growth were clearlyseen in Fig. 3共b兲. On the contrary, the facetlike structure near
the tip was found for nanowires grown at low ambient temperature of 500 ° C in Fig. 3共a兲. In addition, the diameter sizeof the nanowire is much larger than the droplet size even justbelow the droplet. These indicate the occurrence of sidewall
growth for low ambient temperature range. Thus, these experimental results consistently support the second scenario
based on the sidewall growth to interpret the tapering.
Figure 4 shows the schematic illustration of the effect of
ambient temperature on MgO nanowire growth. In the low
ambient temperature range, the adatoms diffused from thesurface contribute not only to the VLS growth but also to thesidewall growth, resulting in the tapering since near the bottom the sidewall growth tends to be enhanced due to thedistribution of diffused adatoms. On the other hand, in thehigh ambient temperature range, the diffusion flux of adatoms is large enough to reach the tip, resulting in the dominance of VLS growth. This physical picture also explains
why the high temperature grown nanowires showed the catalyst diameter dependent longer and untapered morphologies.
The square-rod-like structures of nanowires grown at lower
ambient temperature can be also understood in terms of theoccurrence of the sidewall growth at the bottom, since MgO
is a rocksalt crystal structure. These experimental results
demonstrate the crucial role of the ambient temperature tocontrol arbitrarily the morphologies of oxide nanowires via
VLS mechanism. The present understanding of the underlying mechanism would also contribute to further controllability of complex oxide nanowire VLS growth and the potential
applications.
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