JOURNAL OF APPLIED PHYSICS 101, 124304 共2007兲

Epitaxial growth of MgO nanowires by pulsed laser deposition
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 24 January 2007; accepted 4 May 2007; published online 21 June 2007兲
We fabricated single-crystalline MgO nanowires epitaxially grown on MgO single crystal substrateusing the Au catalyst-assisted pulsed laser deposition 共PLD兲. Controlling appropriately the amountof Au catalyst and the substrate temperature was found to be crucial for the MgO nanowire growthusing the catalyst-assisted PLD. In addition, 共100兲 oriented MgO nanowires were epitaxially grownon 共100兲, 共110兲, and 共111兲 oriented substrates, allowing the limited growth directions. The possible
growth mechanism of MgO nanowires in PLD is discussed. This feasibility of PLD for fabricating
MgO nanowires would contribute to incorporating the rich functionalities of various transition metaloxides into nanowires via in situ construction of heterostructures in oxide nanowires. © 2007
American Institute of Physics. 关DOI: 10.1063/1.2748625兴
One-dimensional nanostructures have become attractive
and being actively investigated since they are promisingnanoscale building blocks for exploring nanoscale devices
including electronics,1–3 photonics,4,5 and ultrasensitive biomolecule sensors.6,7 Mainly, conventional semiconductors
including silicon and its compounds have been focused on as
the nanowire materials. Integrating more functionality into
the nanowires by using other functional materials would be
fascinating for advanced nanowire devices in the future. For
instance, transition metal oxides are well known to exhibit
the rich variation of the physical properties, such as superconductivity, ferromagnetism, and ferroelectricity.8–10 Suchrich functionalities of oxide materials might expand the application range of nanowire devices.11–13 As to the synthesisof oxide materials, pulsed laser deposition 共PLD兲 has been
one of the most powerful techniques to fabricate various
functional oxide thin films,14,15 nonequilibrium oxides,16 andthe heterostructures.17 Since the interface of oxide heterostructures plays a crucial role on the physical properties ofthin films,17,18 in situ fabrication of such oxide heterostructures in nanowires is strongly desired to avoid the degradation of oxide interfaces due to the atmospheric exposure.
Thus the feasibility of PLD technique to fabricate the diverse
oxide nanowires will contribute to the integration of richoxide functionalities into nanowires via in situ constructionof oxide heterostructures in nanowires.
Magnesium oxide 共MgO兲 might find potential applications including catalysis, additives in refractory, paint, andinsulating layers in tunnel magnetoresistance junctions,19,20
and herein more importantly as single crystal substrates fortransition metal oxide thin film growth due to the small lattice mismatch with many transition metal oxides including
magnetite,21 other spinel ferrites,22 and perovskite oxides.23
As such the availability of MgO nanowires using PLD willallow the in situ integration of functional oxide materials into
the nanowires. MgO nanowires, in fact, have been mainly
fabricated by catalysis-assisted chemical-vapor depositiona兲Electronic mail: yanagi32@sanken.osaka-u.ac.jp

0021-8979/2007/101共12兲/124304/4/$23.00

共CVD兲 technique and other techniques in the literature,24–28
although the feasibility of PLD technique for fabricating oxide nanowires including ZnO, which can be grown even in
the absence of a metal catalyst due to the anisotropy of crystal growth, has been demonstrated.29–31 On the contrary there
seem to be no reports concerning with MgO nanowires fabricated by PLD. In addition, the detailed knowledge as to thecontrollability of MgO nanowire morphologies, including thesize and the growth direction, is scarce. Herein we report the
fabrication of epitaxially grown MgO nanowires using the
Au catalyst-assisted PLD.
MgO nanowires were grown on MgO 共100兲 single crystal substrate by Au catalyst-assisted PLD. Au catalyst was
patterned on the 5 ⫻ 5 mm2 substrate by sputtering withmetal mask. The thickness of Au catalyst was controlledfrom 1 to 10 nm by changing the sputtering time. MgO
single crystal was used as a source of Mg species vapor. ArF
excimer laser 共␭ = 193 nm兲 was used for the laser ablation.
The laser energy, the repetition rate, and the oxygen pressure
were set to be 40 mJ, 10 Hz, and 1 Pa, respectively. The
substrate temperature for growth was varied from
400 to 800 ° C. The typical background pressure was
10−4 Pa. Prior to the laser ablation, the Au patterned MgO
共100兲 substrate was preheated at the growth temperature for10 min. The substrate was placed 30 mm away from the target. The laser ablation was performed for 60 min. Subsequently the samples were cooled down to room temperaturein 30 min. The nanowire morphology was characterized byfield emission scanning electron microscopy 共FESEM兲共JEOL JSM-6330FT兲. High-resolution transmission electron
microscopy 共TEM兲 共JEOL JEM-3000F兲 coupled with energy
dispersive spectroscopy 共EDS兲 was used to evaluate the diameter, the crystallinity, and the composition of the fabricated nanowires. Samples for TEM were prepared by placinga drop of the sample suspension on a copper microgrid
共JEOL 7801-11613兲. TEM measurements were performed at
the accelerating voltage of 300 kV. The crystal structure of
the as-synthesized products was analyzed by x-ray diffraction 共XRD兲.

101, 124304-1© 2007 American Institute of Physics

Downloaded 21 Jun 2007 to 133.1.58.105. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

124304-2

Nagashima et al.

J. Appl. Phys. 101, 124304 共2007兲

FIG. 1. FESEM images of MgO nanowires on MgO
共100兲 single crystal substrate. In images 共a兲, 共b兲, and共c兲, the nanowires were grown under 800 ° C substratetemperature and 1 nm Au film thickness. 共a兲 Patternedarea of Au on MgO 共100兲 single crystal substrate. The
scale bar is 500 ␮m. 共b兲 Morphologies of MgO nanowires in central area “b” in 共a兲. The scale bar is 100 nm.
共c兲 Morphologies of MgO nanowires in boundary area“c” in 共a兲. The scale bar is 100 nm. 共d兲 Effect of theamount of Au catalyst on the morphologies of deposited
surface. The Au catalyst thickness was 10 nm and thesubstrate temperature was 800 ° C. The scale bar is
1 ␮m. 共e兲 Dependence of the substrate temperature on
the morphologies of deposited surface. The substratetemperature was 400 ° C and the Au catalyst thickness
was 1 nm. The scale bar is 100 nm.

Figure 1 shows the typical FESEM images of the sample
morphology. The growths were performed under 800 ° C
substrate temperature and 1 nm Au film thickness. The Au
patterned area on the MgO substrate can be seen in Fig. 1共a兲.
The magnified image near the center of the pattered area isshown in Fig. 1共b兲. Nanowire structures were clearly observed, and the size was found to be relatively homogeneousin the image. The magnified image near the boundary of thepatterned area is shown in Fig. 1共c兲. It can be seen that the
nanowires were only grown for Au patterned area, indicating
the important role of Au catalyst on the nanowire growth. In
addition, most nanowires were grown perpendicular to thesubstrate, inferring the epitaxial growth of nanowires. The
average nanowire length grown for 60 min was 150 nm. It isnoted that the appearance of MgO nanowires is sensitive toseveral experimental conditions. For instance, when varying
the thickness of Au catalyst from 1 up to 10 nm, the presenceof nanowires has not been observed at all above 5 nm, as
shown in Fig. 1共d兲, in which Au agglomerates only wereobserved. Such critical dependence on the nanowire growth
was also found for the variation of the substrate temperature.
Figure 1共e兲 shows the image when the substrate temperature
was set to be below 450 ° C. Clearly there were no observable nanowires due to such lower substrate temperature.
Thus it is crucial to control appropriately the ambient conditions, including mainly the amount of Au catalyst and thesubstrate temperature, for the nanowire growth using PLD.
Figure 2共a兲 shows the 2␪-␪ scan data of XRD for MgO nanowires grown on MgO 共100兲 single crystal substrate under
800 ° C substrate temperature and 1 nm Au film thickness.

Only one diffraction peak corresponding to the MgO 共200兲
plane was detected, indicating the epitaxy of MgO nanowire
growth on MgO 共100兲 single crystal substrate. Figure 2共b兲shows the nanowire length versus the growth time for nanowires grown under 800 ° C substrate temperature and 1 nm
Au film thickness. It can be seen that the nanowire length
increases with increasing growth time within the investigatedtime scale. Although the average diameter of nanowires wasapproximately 15– 20 nm in the FESEM images, the values
are overestimated due to the use of platinum coating thatprevents charging in SEM measurements. high-resolution
TEM 共HRTEM兲-EDS analysis was performed to obtain moredetailed information in the context of the nanowires, such as
the exact size, the crystallinity, and the composition. Figure
3共a兲 shows the HRTEM image of the MgO nanowires. The
presence of the catalyst droplet on the wire tip can be clearlyseen in the image. As to the crystallinity of the nanowires,the lattice fringes of nanowires were observed in the HRTEM image, as shown in the inset figure. It reveals that the
spacing of 0.21 nm between adjacent lattice planes corresponds to the distance between 共200兲 crystal planes of MgO,
indicating the single crystal nature of the MgO nanowires. In
addition, the selected area electron diffraction 共SAED兲 pattern of the MgO nanowire reveals that the nearest diffraction
spot to the center corresponds to the 共200兲 plane, as shown in
Fig. 3共c兲. To confirm the compositions of both droplets andwires, EDS analysis was performed at each area, as shown in
Figs. 3共d兲 and 3共e兲. Cu signals in these EDS spectra are
originated from the TEM grids. The droplets are mainlycomposed of Au, and the wires are formed by Mg and O with

FIG. 2. XRD data of MgO nanowires
grown on 共100兲 substrate and thenanowire length vs the growth time.
The nanowires were grown under
800 ° C substrate temperature and1 nm Au film thickness. 共a兲 XRD dataof MgO nanowires grown on 共100兲
substrate. The inset shows the zoom-in
view of the primary peak at 共200兲
plane. 共b兲 Growth time dependence on
nanowire length.

Downloaded 21 Jun 2007 to 133.1.58.105. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

124304-3

Nagashima et al.

J. Appl. Phys. 101, 124304 共2007兲

FIG. 3. HRTEM images of MgO
nanowires and the EDS data. The
nanowires were grown under 800 ° C
substrate temperature and 1 nm Au
film thickness. 共a兲 HRTEM image of
MgO nanowires. The scale bar is
20 nm. The inset figure shows the lattice fringe image. 共b兲 HRTEM imageof small MgO nanowires. The scalebar is 5 nm. 共c兲 SAED pattern of MgO
nanowires. 共d兲 EDS data at “c*” position in 共a兲. 共e兲 EDS data at “d*” position in 共a兲.

the ratio of Mg to O approximately equal to 1:1. In addition,
as can be seen in Fig. 3共b兲, the average diameter size could
be controlled down to 3 – 5 nm by manipulating the Au droplet size. The preferential 关100兴 growth observed in MgO
nanowires might be principally extended to control thegrowth direction from the substrate. Figure 4 shows the orientation effect of MgO substrate on the growth direction of
MgO nanowires using MgO 共100兲, 共110兲, and 共111兲 oriented
single crystal substrates. Figures 4共a兲 and 4共b兲 show thetilted FESEM images for 共100兲 and 共110兲 substrates, respectively, and Fig. 4共c兲 shows the top-view image of 共111兲 substrate. The growth direction of fabricated nanowires strongly
depends on the substrate orientation. Clearly, the epitaxial
growth of MgO nanowires along the 关100兴 direction limits
the growth direction of nanowires for each oriented sub-strate. Note that nanowires more than 90% follow the growth
regime. In addition, such epitaxitial growths of 关100兴 oriented MgO nanowires grown on 共110兲 and 共111兲 substrates
were also confirmed by XRD measurements, as shown in
Figs. 4共d兲 and 4共e兲. Considering the experimental results including the presence of catalyst on the wire tip, the vaporliquid-solid 共VLS兲 mechanism seems to be responsible for
the MgO nanowire growth in PLD.32 Within the frameworkof the VLS mechanism,33 a supersaturation is an important
process for the nanowire growth. When varying the amountof catalyst and the substrate temperature, the degree of supersaturation within the catalyst might be altered. In addition, the diffused adatoms on the substrate surface rather than
impinging species play an important role on the VLS
growth.34 These might explain why the MgO nanowires can

FIG. 4. Orientation effect of MgOsubstrate on the growth direction of
MgO nanowires. The nanowires weregrown under 800 ° C substrate temperature and 1 nm Au film thickness.
共a兲 Tilted image of MgO nanowires
grown on 共100兲 substrate. 共b兲 Tilted
image of MgO nanowires grown on
共110兲 substrate. 共c兲 Top-view image of
MgO nanowires grown on 共111兲 substrate. 共d兲 XRD data of MgO nanowires grown on 共110兲 substrate. The
inset shows the zoom-in view of the
primary peak at 共220兲 plane. 共e兲 XRD
data of MgO nanowires grown on
共111兲 substrate. The inset shows thezoom-in view of the primary peak at
共111兲 plane.

Downloaded 21 Jun 2007 to 133.1.58.105. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

124304-4

J. Appl. Phys. 101, 124304 共2007兲

Nagashima et al.

be grown in catalyst-assisted PLD, and the critical dependencies on the nanowire growth exist when varying the amountof catalyst and the substrate temperature. For example, increasing catalyst thickness results in increasing catalyst droplet size.35 If the diffusion length of adatoms is solely dependent on the substrate temperature at constant ambientpressure, the diffusion flux of adatoms into the individual
catalyst droplet is enhanced with decreasing the droplet sizeat constant substrate temperature, which varies the degree ofsupersaturation within the catalyst droplet. This might explain the crucial dependence of the amount of catalyst on the
MgO nanowire growth in terms of the variation of supersaturation. As to the substrate temperature effects, there are two
possible factors affecting the MgO nanowire growth. One isrelated to the phenomenon of melting Au catalyst, since themelting temperature of Au bulk is 1064 ° C, which is muchhigher than the substrate temperature used. Clearly decreasing the substrate temperature prevents the melting of Au
catalyst, which strongly affects the VLS growth via the absence of liquid phase. The other factor might be the diffusion
flux of adatoms. Since increasing the substrate temperatureenhances the surface diffusion of adatoms, the diffusion flux
into the catalyst droplet decreases with decreasing the substrate temperature. This also varies the degree of supersaturation within the droplet. Although the present understandingof the MgO nanowire growth is not comprehensive, above
qualitative interpretations would contribute to the further understanding of oxide nanowire growth in PLD and also controlling oxide nanowire morphologies.
In conclusion, we have fabricated epitaxially grown
MgO nanowires using Au catalyst-assisted PLD technique.
As to the properties of MgO nanowires, the well-crystallized
structures, the average size down to 5 nm, and the stoichiometry were confirmed by HRTEM-EDS. The preferential
epitaxial growth of 关100兴 direction could be extended to control the growth direction of nanowires by varying the substrate orientation. The feasibility of PLD technique for fabricating MgO nanowires will allow the integration of richfunctionalities of many transition metal oxides into oxide
nanowires via in situ fabrication of oxide heterostructures.
ACKNOWLEDGMENTS

The authors would like to thank the Ministry of Education, Culture, Sports, Science and Technology of Japan for
funding and supporting this project though the Center of Excellence 共COE兲 Program. The authors also acknowledge M.
Kanai for constructive advices and T. Ishibashi for his invaluable technical support. One of the authors 共K.N.兲 greatly
thanks the Micron Technology for scholarship.

1

F. Patolsky, B. D. Timko, G. Yu, Y. Fang, A. B. Greytak, G. Zheng, and C.
M. Lieber, Science 313, 1100 共2006兲.
2
J. Xiang, W. Lu, Y. Hu, Y. Wu, H. Yan, and C. M. Lieber, Nature 共London兲
441, 489 共2006兲.
3
R. S. Friedman, M. C. McAlpine, D. S. Ricketts, D. Ham, and C. M.
Lieber, Nature 共London兲 434, 1085 共2005兲.
4
O. Hayden, R. Agarwal, and C. M. Lieber, Nat. Mater. 5, 352 共2006兲.
5
C. J. Barrelet, J. Bao, M. Lončar, H. G. Park, F. Capasso, and C. M.
Lieber, Nano Lett. 6, 11 共2006兲.
6
F. Patolsky, G. Zheng, and C. M. Lieber, Anal. Chem. 78, 4260 共2006兲.
7
G. Zheng, F. Patolsky, Y. Cui, W. U. Wang, and C. M. Lieber, Nat. Biotechnol. 23, 1294 共2005兲.
8
W. Ramadan et al., Phys. Rev. B 72, 205333 共2005兲.
9J. P. Locquet, J. Perret, J. Fompeyrine, E. Mäechler, J. W. Seo, and G. vanTendeloo, Nature 共London兲 394, 453 共1998兲.
10
B. S. Kwak, A. Erbil, B. J. Wilkens, J. D. Budai, M. F. Chisholm, and L.
A. Boatner, Phys. Rev. Lett. 68, 3733 共1992兲.
11
B. Lei, C. Li, D. Zhang, S. Han, and C. Zhou, J. Phys. Chem. B 109,
18799 共2005兲.
12
C. Li, B. Lei, Z. Luo, S. Han, Z. Liu, D. Zhang, and C. Zhou, Adv. Mater.
共Weinheim, Ger.兲 17, 1548 共2005兲.
13
D. Zhang, Z. Liu, S. Han, C. Li, B. Lei, M. P. Stewart, J. M. Tour, and C.
Zhou, Nano Lett. 4, 2151 共2004兲.
14
T. Kanki, T. Yanagida, B. Vilquin, H. Tanaka, and T. Kawai, Phys. Rev. B
71, 012403 共2005兲.
15
T. Yanagida, H. Tanaka, T. Kawai, E. Ikenaga, M. Kobata, J. J. Kim, andK. Kobayashi, Phys. Rev. B 73, 132503 共2006兲.
16
H. Ryoken, N. Ohashi, I. Sakaguchi, Y. Adachi, S. Hishita, and H. Haneda,
J. Cryst. Growth 287, 134 共2006兲.
17
T. Kanki, Y. G. Park, H. Tanaka, and T. Kawai, Appl. Phys. Lett. 283,
4860 共2003兲.
18
Y.-G. Park, T. Kanki, H.-Y. Lee, H. Tanaka, and T. Kawai, Solid-StateElectron. 47, 2221 共2003兲.
19
H. Tsuji, F. Yagi, H. Hattori, and H. Kita, J. Catal. 148, 759 共1994兲.
20
Y. M. Lee, J. Hayakawa, S. Ikeda, F. Matsukura, and H. Ohno, Appl. Phys.
Lett. 89, 042506 共2006兲.
21
D. M. Lind, S. D. Berry, G. Chern, H. Mathias, and L. R. Testardi, Phys.
Rev. B 45, 1838 共1992兲.
22
X. ZuO, A. Yang, S. D. Yoon, J. A. Christodoulides, V. G. Harris, and C.
Vittoria, J. Appl. Phys. 97, 10G103 共2005兲.
23
I. B. Misirlioglu, S. P. Alpay, F. He, and B. O. Wells, J. Appl. Phys. 99,
104103 共2006兲.
24
H. W. Kim and S. H. Shim, Chem. Phys. Lett. 422, 165 共2006兲.
25
C. Tang, Y. Bando, and T. Sato, J. Phys. Chem. B 106, 7449 共2002兲.
26
Y. Yin, G. Zhang, and Y. Xia, Adv. Funct. Mater. 12, 293 共2002兲.
27
H. Lu, H. Sun, G. Li, C. Chen, D. Yang, and X. Hu, Ceram. Int. 31, 105
共2005兲.
28
X. Wang and D. Xue, Mater. Lett. 60, 3160 共2006兲.
29
T. Nobis, E. M. Kaidashev, A. Rahm, M. Lorenz, J. Lenzner, and M.
Grundmann, Nano Lett. 4, 797 共2004兲.
30
M. Lorenz et al., Appl. Phys. Lett. 86, 143113 共2005兲.
31
S. Choopun, H. Tabata, and T. Kawai, J. Cryst. Growth 274, 167 共2005兲.
32
J. Zhan, Y. Bando, J. Hu, and D. Golberg, Inorg. Chem. 43, 2462 共2004兲.
33
J. Hu, T. W. Odom, and C. M. Lieber, Acc. Chem. Res. 32, 435 共1999兲.
34
V. G. Dubrovskii, N. V. Sibirev, G. E. Cirlin, J. C. Harmand, and V. M.
Ustinov, Phys. Rev. E 73, 021603 共2006兲.
35
V. G. Dubrovskii, I. P. Soshnikov, G. E. Cirlin, A. A. Tonkikh, Yu. B.
Samsonenko, N. V. Sibirev, and V. M. Ustinov, Phys. Status Solidi B 241,
R30 共2004兲.

Downloaded 21 Jun 2007 to 133.1.58.105. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp