Published on Web 03/22/2008

Effect of the Heterointerface on Transport Properties of in
Situ Formed MgO/Titanate Heterostructured Nanowires
Kazuki Nagashima, Takeshi Yanagida,* Hidekazu Tanaka, Shu Seki, Akinori Saeki,
Seiichi Tagawa, and Tomoji Kawai
Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki,
Osaka 567-0047, Japan
Received January 16, 2008; E-mail: yanagi32@sanken.osaka-u.ac.jp

Abstract: Heterostructured transition metal oxide nanowires are potential candidates to incorporate richfunctionalities into nanowire-based devices. Although the oxide heterointerface plays a crucial role indetermining the physical properties, the effects of the heterointerface on the oxide nanowire’s properties
have not been clarified. Here we investigate for the first time the significant role of the heterointerface indetermining the transport properties of well-defined MgO/titanate heterostructured nanowires by combining
a technique for in situ formation of a oxide heterointerface and microwave conductivity measurement.
Variation of the heterointerface strongly affects the nanowire’s transport properties due to the crystallinity
and the atomic interdiffusion at the oxide heterointerface. Thus, the precise in situ formation of a welldefined heterointerface is crucial to create oxide heterostructured nanowires with the desired transport
properties.

Introduction

Transition metal oxide nanowires are potential candidates toincorporate desirable physical properties, including superconductivity, ferromagnetism, and ferroelectricity, into nanowirebased devices.1–4 Since such fascinating oxide materials typicallyhave multiple components,1–4 one-dimensional nanostructures
composed of such multicomponent oxides are highly desired.
However, there is a fundamental limitation to fabricating
nanowires of various transition metal oxides, resulting from onedimensional crystal growth mechanisms, such as the vapor–liquid–solid (VLS) mechanism. 5 One of the ways to overcome
this critical issue is to utilize a core/shell heterostructured oxide
nanowire,6–8 which allows us to explore a wide range of oxidematerials. In oxide heterostructures, the heterointerface is wellknown to play a crucial role in determining the physicalproperties.9–11 Thus, fabricating well-defined heterointerfaces
in core/shell heterostructured nanowires is also strongly desir-able. In situ formation of core/shell heterostructured nanowires
would be a powerful method to prevent atmospheric exposure.
However, such in situ formation of core/shell heterostructuredoxide nanowires, systematic control of the growth, and the effectof the formed heterointerface on the transport properties havenot been reported so far. Previously, we have demonstrated thefeasibility and the controllability of magnesium oxide (MgO)
nanowires formed by using pulsed laser deposition (PLD).12–15
This in principle allows us to form in situ core/shell heterostructured oxide nanowires. Titanate is a highly attractive
material for diverse applications, including uses in photocatalysts,16 oxide electronics,17 and nonvolatile memory devices.18
Here we investigate the in situ formation of MgO/titanate
heterostructured oxide nanowires and the effect of the heterointerface on the transport properties. Our results demonstrate
the significant role of the heterointerface variation on the(1) Ramadan, W.; Ogale, S. B.; Dhar, S.; Fu, L. F.; Shinde, S. R.;
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(7) Zhang, D.; Liu, Z.; Han, S.; Li, C.; Lei, B.; Stewart, M. P.; Tour,J. M.; Zhou, C. Nano Lett. 2004, 4, 2151–2155.
(8) Li, C.; Lei, B.; Luo, Z.; Han, S.; Liu, Z.; Zhang, D.; Zhou, C. AdV.
Mater. 2005, 17, 1548–1553.
(9) Nagashima, K.; Yanagida, T.; Tanaka, H.; Kawai, T. J. Appl. Phys.
2006, 100, 063714.

(10) Nagashima, K.; Yanagida, T.; Tanaka, H.; Kawai, T. Phys. ReV. B
2006, 74, 172106.
(11) Nagashima, K.; Yanagida, T.; Tanaka, H.; Kawai, T. J. Appl. Phys.
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(12) Nagashima, K.; Yanagida, T.; Tanaka, H.; Kawai, T. J. Appl. Phys.
2007, 101, 124304.
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2007, 90, 233103.
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2007, 91, 016502.
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10.1021/ja800367a CCC: $40.75  2008 American Chemical SocietyTransport Properties of MgO/Titanate Nanowires

ARTICLEStransport properties of heterostructured oxide nanowires toward
the electronic device applications.
Experimental Section
Fabrication of MgO Core Nanowires. MgO nanowires weregrown on MgO (100) single-crystal substrate by the Au catalystassisted PLD method.12–15 Prior to the nanowire growth, a Au thinfilm was deposited on the 5 mm × 5 mm MgO substrate. The
background pressure of the PLD chamber was set to 10-5 Pa. An
ArF excimer laser (Lamda-Physik COMPex 102, λ ) 193 nm) wasused for the laser ablation. The MgO single-crystal substrate wasused as a source of Mg species vapor. The laser energy and therepetition rate were set to 40 mJ and 10 Hz, respectively. The
distance between the substrate and the target was 30 mm. Oxygengas was introduced into the chamber at a constant ambient pressureof 10 Pa. Prior to the laser ablation, the Au-coated MgO (100)single-crystal substrate was preheated to 800 °C and kept at thattemperature for 10 min. After 180 min of deposition, the samples
were cooled to room temperature within 30 min under 10 Pa oxygenpressure.
Titanate Shell Layer Growth. A titanate shell layer was grownon vertical arrayed MgO nanowires. The same system used for MgO
nanowire growth was used for the shell layer growth. A TiO2 pellet
was used as the target, which was obtained by milling TiO2 (99.9%
pure) powder for 1 h and then sintering at 1000 °C for 24 h. After
sintering, the samples were reground for 3 h and then calcined at
500 °C for 5 h. The repetition rate was set to 3 Hz. The laser energy,
substrate-to-target distance, oxygen pressure, and substrate temperature were 5–50 mJ, 30–45 mm, 10-3-10 Pa, and room
temperature to 800 °C, respectively. The deposition rate for shelllayer growth was ∼0.4 Å/min. After the deposition, the samples
were cooled to room temperature for 30 min.
Sample Characterizations. To characterize the nanowire morphology, field emission electron microscopy (FESEM, JEOL JSM6330FT) at an accelerating voltage of 30 kV was used. Prior to the
FESEM observation, Pt was deposited on the samples to prevent
charging. The thickness of Pt was nearly 5 nm. High-resolution
transmission electron microscopy (HRTEM, JEOL JEM-3000F)
coupled with energy-dispersive spectroscopy (EDS) was used toevaluate the diameter, crystallinity, local crystal structure, andcomposition of the fabricated nanowires. Samples for HRTEM wereprepared by placing a drop of the sample suspension on a copper
microgrid (JEOL 7801-11613). HRTEM measurements were performed at an accelerating voltage of 300 kV.
Microwave Conductivity Measurement. For microwave conductivity measurement, the microwave frequency and power wereset at ∼9.1 GHz and 3 mW, respectively, so that the motion of
charge carriers was no disturbed by the low electric field of the
microwave. The absorption of microwave power by the samples
was monitored as changes in the reflected microwave from aresonant cavity (Q ≈ 2500) with the samples and measured as afunction of the frequency. All the above experiments were carriedout at room temperature. The conductivity (∆σ) of the samples isrelated to the reflected microwave power at the resonant frequency(∆Pr/Pr) via〈∆σ〉 )

1 ∆Pr
A Pr

(1)

where A is a sensitivity factor. The other details of the apparatus
are described elsewhere. The absolute value of conductivity wascalibrated relative to the value of a thin film grown epitaxially on
the MgO substrate, where carrier density and mobility had been
determined by a Hall effect measurement.
Results and Discussion

Figure 1a shows the FESEM image of MgO nanowires grown
on a MgO (100) single-crystal substrate. All nanowires weregrown epitaxially on the substrate. These nanowires are typicallyFigure 1. Characteristics of the MgO nanowire template. (a) FESEM imageof MgO nanowires grown on MgO (100) substrate. All nanowires weregrown epitaxially on the substrate. (b) XRD pattern of MgO nanowires
fabricated on MgO (100) single crystal substrate. (c) HRTEM image of anindividual MgO nanowire. Inset: SAED pattern of the MgO nanowire,
identifying the [100] preferential growth direction.

Figure 2. (a) Schematic illustration of the in situ heterostructured nanowire

formation using PLD. (b,c) FESEM images of the MgO/titanate heterostructured nanowires grown at laser energies of 15 and 50 mJ, respectively.
All heterostructured nanowires were grown at 500 °C and 10-3 Pa.

several micrometers long and around 10–20 nm in diameter.
X-ray diffraction (XRD) measurement shows only a MgO (200)peak due to the preferential [100] growth of MgO nanowires,as shown in Figure 1b.12 The preferential crystal growth, singlephase crystallinity, and smooth sidewall were confirmed by
HRTEM, as shown in Figure 1c.
A titanate shell layer was deposited on these MgO nanowires
by an in situ PLD technique. A schematic of the shell layer
formation is illustrated in Figure 2a. Figure 2b,c shows the
FESEM images of the MgO/titanate heterostructured nanowires
when the ablated particle flux is varied; the nanowires were
deposited with laser energies of 15 and 50 mJ, respectively. A
clear dependence of the morphologies on the ablated particle
flux can be seen. When the laser energy was set to 15 mJ, a
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Nagashima et al.

Figure 3. Temperature dependence on the morphologies of MgO/titanate heterostructured nanowires. HRTEM images show the heterostructured nanowires
grown at (a) room temperature, (b) 300 °C, (c) 500 °C, and (d) 800 °C. All shell formations were performed at 10-3 Pa oxygen pressure and 15 mJ laser
energy. The lower figures are magnified images of the heterointerface and schematic illustrations of the heterointerface variations. Inset in (d) is the SAED
pattern of nanowires grown at 800 °C.

relatively smooth covering surface was observed, as shown in
Figure 2b. Note that shell layers were not observable below alaser energy of 10 mJ due to the absence of adatoms on thesurface. On the other hand, a rough surface and grain-like shelllayers were found when the laser energy was increased above
20 mJ. In addition, the difference between the bottom diameterand the tip diameter increases with increasing laser energy,
indicating the preferential crystal growth and the nucleation on
the tip. It is noted that these trends did not depend on thesubstrate temperature, repetition rate, and deposition time, but
strongly depended on both the distance between the target andthe substrate and the oxygen pressure, which strongly affectthe ablated particle flux. This critical dependence of the
nanowire morphologies on ablated particle flux might be due
to nucleation phenomena of the ablated species. Generally, when
the ablated particle flux is increased, the collision rate between
the particles and the probability of nuclear generation increase.
Once the nucleus generates at the tip, the ablated particles
preferentially assemble the nucleus. As such, the increased
ablated particle flux might enhance the preferential growth at
the tip, resulting in the large size difference between the tip
and the bottom. Thus, controlling the ablated particle flux is
crucial to achieve the homogeneous shell covering.
Figure 3 shows the HRTEM images of the MgO/ titanate
heterostructured nanowires when the growth temperature wasvaried from room temperature up to 800 °C at a laser energyof 15 mJ. The core diameter and the shell thickness weretypically 10–20 and 5 nm, respectively. Changing the deposition
time allows easy control of the shell thickness. There is a wide
variation of the heterointerface and the shell layer crystallinity
when the temperature is varied. Increasing the growth temperature drastically changes the crystallinity of shell layer from(a) the amorphous phase, (b) the polycrystalline-like structure,(c) the single-crystal core/shell heterostructures, and (d) the
mixed crystal structures. The variation from the amorphous
phase to the well-defined core/shell heterostructures can be
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understood in terms of the enhancement of crystallinity at theheterointerface with increasing temperature. Figure 4a shows
the XRD data of nanowires when the growth temperature isvaried for the shell layer deposition. It can be seen that thetitanate crystallinity changes from the amorphous phase at room
temperature to epitaxial (110)-oriented rutile structures at 500°C. The epitaxial growth is illustrated in Figure 4b. In fact, thesetrends basically agree with the HRTEM results in Figure 3. Suchvariation of crystallinity with varying growth temperature wasfound on titanate thin films grown on MgO (100) substrate (seeFigure S1 in the Supporting Information). However, the presenceof polycrystalline structure at 300 °C differs from the case of
the thin films, since the thin films grown at 300 °C showed theepitaxial growth of mixed (110)-oriented rutile structure and(100)-oriented anatase structure. This discrepancy might be dueto the limitation of surface migration on the nanowire sidewall.
The presence of (110)-oriented rutile structure was no longer
observable when the growth took place at 800 °C, indicating achange of crystal structure. To identify the crystal structures
grown at high temperature, fine XRD scans were performed,
as shown in Figure 4c. A shoulder-like peak was observed below
the MgO (200) peak, and the lattice constant around the peakis consistent with possible (Mg,Ti)3O4 spinel structures. The
selected area electron diffraction (SAED) patterns for nanowires
formed at 800 °C agreed with such cubic spinel structures, as
shown in Figure 3. This implies Mg diffusion into the shelllayers during shell layer formation. In addition, as shown in
Figure 4d, the EDS measurement demonstrates that the atomic
ratio of Mg/Ti is nearly 1:1, indicating the formation of
Mg1.5Ti1.5O4 spinel structure. The Mg diffusion length in thissystem is over 200 nm (see Figure S2 in the Supporting
Information) and large enough compared with the shell thicknessof 5 nm. These results clearly indicate that the crystal structureof nanowires formed at 800 °C is a mixed spinel structure.
The above results highlight the wide variation of theheterointerface in oxide nanowires when the ambient atmosphereTransport Properties of MgO/Titanate Nanowires

ARTICLES

Figure 4. (a) XRD patterns of the MgO/titanate heterostructured nanowires with varying formation temperature. (b) Schematic of the epitaxial growth ofrutile TiO2 on MgO. The TiO2 (110) plane and the MgO (100) plane match well by the long-range epitaxial rule of 6:13. (c) Fine XRD pattern of MgO/
titanate heterostructured nanowires grown at 800 °C. (d) EDS data of the MgO/titanate heterostructured nanowires grown at 800 °C. All shell formations
were performed at 10-3 Pa oxygen pressure.

is varied. In principle, such heterointerface variation should
affect the charge carrier transport properties of heterostructured
nanowires. One of critical issues in measuring the transport
properties of nanowires is the contact with electrodes at thenanoscale. Microwave conductivity measurement allows us toinvestigate the transport properties of such nanoscale materials
without electrodes and contact problems.19,20 Figure 5 shows
the results of microwave conductivity measurements for heterostructured oxide nanowires when the growth temperature wasvaried from room temperature up to 800 °C. The microwave
conductivity measurements were performed at 300 K.
There are two major trends in the transport when thegrowth temperature was varied. First, the resistivity decreased
when the growth temperature increased below 400 °C. The
enhancement of electrical conductivity below 400 °C can beinterpreted in terms of the crystal variation from theamorphous phase to the single-crystal core/shell heterostructures, as can be seen in the HRTEM images (Figure 3). Such
reduction of resistivity with increasing growth temperaturefrom room temperature to 400 °C was also found in titanate
thin-film growth, as shown in Figure 5d (see Figure S3 in
the Supporting Information). Thus, the enhancement ofcrystallinity at the heterointerface is responsible for theobserved trend in heterostructured nanowires grown below
400 °C. However, it is noted that the resistivity of nanowires
grown at 300 °C is rather high, even in comparison with thetrend in the case of thin films. This discrepancy is due to thedifference in crystallinity between the nanowires and the thinfilms. As shown in Figure 3, the heterostructured nanowires
showed polycrystalline-like structure, whereas the thin films
were grown epitaxially. It should be emphasized that the(19) Yamamoto, Y.; Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki,
S.; Tagawa, S.; Taniguchi, M.; Kawai, T.; Aida, T. Science 2007, 314,
1761–1764.
(20) Hoofman, Romano, J. O. M.; de Haas, Matthijs, P.; Siebbeles, Laurens,
D. A.; Warman, John, M. Nature 1998, 392, 54–56.

resistivity of the heterostructured nanowires grown at 400°C is comparable to that of the thin films, as shown in Figure
5c. This indicates the high quality of the shell layer in theheterostructured nanowires achieved by controlling appropriately the ambient atmosphere. Thus, the above results
highlight that the crystal difference at the heterointerface
strongly affects the transport properties of heterostructured
nanowires.
Second, above a growth temperature of 400 °C, theresistivity increased through the minimum with increasing
growth temperature. This can be understood in terms of theformation of mixed Mg2-xTi1+xO4 spinel structures at relatively high temperatures, since Mg2TiO4 is an insulator.21
The presence of mixed Mg2-xTi1+xO4 spinel structures has
been discussed (Figure 4). However, the resistivity ofheterostructured nanowires grown at 800 °C is rather low
when compared with the trends of the thin films (Figure 5d).
This would be due to the composition of mixed Mg2-xTi1+xO4
spinels, because increasing x results in decreasing resistivity.21 In thin films, Mg atoms can be supplied from singlecrystal substrate, whereas there is a limit of Mg atoms forheterostructured nanowires. As shown in Figure 4d, thecomposition of spinel structure for nanowires formed at 800°C is Mg1.5Ti1.5O4. As such, the composition difference inmixed spinel structures causes the discrepancy between thetransport properties of the heterostructured nanowires andthe thin films. Thus, the nature of the mixing at theheterointerface strongly affects the transport properties of
MgO/titanate core–shell nanowires. The above results highlight that realizing the desired transport properties in welldefined oxide heterostructured nanowires requires the precise(21) Isobe, M.; Ueda, Y. J. Alloys Compd. 2004, 383, 85–88.
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Figure 5. (a,b) Dependence of microwave power, reflected from the cavity of the samples, on the frequency. (a) Entire views of the typical resonance curvein microwave measurement for heterostructured nanowires grown at 500 °C. (b) Magnified views at the resonance frequencies of heterostructured nanowires
when the formation temperature was varied. (c,d) Effects of formation temperature on the resistivity of (c) MgO/titanate heterostructured nanowires and (d)thin films, respectively. All shell formations were performed at 10-3 Pa oxygen pressure.

control of the crystal formation and the atomic interdiffusion
at the oxide heterointerface.

defined heterointerface is crucial to create the desiredtransport properties in oxide heterostructured nanowires.

Conclusion

Acknowledgment. The authors acknowledge M. Kanai forconstructive advice and T. Ishibashi for invaluable technicalsupport. K.N. greatly thanks the Micron Technology forscholarship.

In summary, we investigated the effect of the heterointerface on the transport properties of in situ formed MgO/
titanate heterostructured nanowires by using PLD. The in situ
formation of the heterointerface was found to be strongly
dependent on both the ablated particle flux and the growthtemperature. By controlling appropriately the ambient atmosphere during the in situ heterostructure formation, it ispossible to fabricate a well-defined oxide heterointerface innanowires, which is comparable with thin films in terms ofthe transport properties. In addition, variations in the heterointerface strongly affect the transport properties via the
crystallinity and the atomic interdiffusion at the oxide
heterointerface. Thus, the precise in situ formation of well-

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Supporting Information Available: Results of the crystal
structures and the transport properties in titanate thin films
deposited on MgO (100) substrate with varying deposition
conditions, including the substrate temperature and the film
thickness. This material is available free of charge via theInternet at http://pubs.acs.org.
JA800367A