APPLIED PHYSICS LETTERS 95, 133110 共2009兲

Specific surface effect on transport properties of NiO/MgO heterostructurednanowires
Keisuke Oka,1 Takeshi Yanagida,1,2,a兲 Kazuki Nagashima,1 Hidekazu Tanaka,1Shu Seki,2,3 Yoshihito Honsho,3 Manabu Ishimaru,1 Akihiko Hirata,1 and Tomoji Kawai1,b兲
1Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki,
Osaka 567-0047 Japan
2PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, 332-0012 Japan
3Division of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita,Osaka, 565-0871 Japan

共Received 21 July 2009; accepted 3 September 2009; published online 30 September 2009兲
NiO heterostructured nanowires are promising building blocks due to the nonvolatile resistive
switching in nanoscale. Here, we report on the noncontact transport measurements of single
crystalline NiO/MgO heterostructured nanowires by utilizing a microwave conductivity method. We
found the substantial discrepancy up to four orders of magnitude between the heterostructured
nanowires and heterothin films on the resistivity when the bulk resistivity increased, whereas thereasonable agreement was found for relatively conductive range. The origin of such huge
discrepancy was interpreted in terms of both the large specific surface area of nanowires and thesurface transport events of insulative NiO. © 2009 American Institute of Physics.
关doi:10.1063/1.3237176兴
One-dimensional nanostructures using transition metaloxides 共TMOs兲 are potential candidates to incorporate thefascinating physical properties, including superconductivity,1ferromagnetism,2 ferroelectric,3 resistive memory effect,4
metal-insulator transition,5 and others, into nanowire-based
devices.6 Among various interesting TMOs, NiO is an antiferromagnetic insulator, and, historically, the insulating nature, which cannot be explained in terms of a classical bandmodel, has been intensively studied to understand the role ofan electron correlation.7 Other investigations as to NiO havedealt with the antiferromagnetic properties including the exchange bias effect,8 sensing gases via the semiconducting
properties9 and the electrochromism utilizing the redox
phenomena.10 Recently, nonvolatile resistive switchingmemory effects in NiO thin films have renewed the interestsin the transport properties of NiO toward the nonvolatilememory device applications.11–13 Creating one-dimensional
NiO nanostructures via self-assembling would be one key
technological step toward nanoscale nonvolatile memory devices using NiO. However, the fabrication of NiO nanowires
had been difficult and unfeasible due to the material
limitation of one-dimensional crystal growth mechanisms including vapor-liquid-solid growth.14,15 One of the ways toovercome this critical issue is to utilize a core/shell heterostructured oxide nanowire, since the method allows us tofabricate a wide range of oxide materials.16–18 Previously, wehave fabricated the MgO/NiO core/shell heterostructured
nanowires19 using the in situ formation technique newly
developed.20 Since both MgO and NiO have the same crystal
structure 共rock-salt兲 and the lattice mismatch is quite small
共0.7%兲, the material combination of MgO and NiO is ideal tocreate the well-defined core/shell heterostructured nanowires.
In addition, we have shown the existence of nonvolatilememory effects within the NiO-based heterostructured nanoa兲

Electronic mail: yanagi32@sanken.osaka-u.ac.jp.
Electronic mail: kawai@sanken.osaka-u.ac.jp.

b兲0003-6951/2009/95共13兲/133110/3/$25.00

wires down to 10 nm range.19 However, the knowledge as tothe transport properties of MgO/NiO heterostructured nanowires is still scarce due to the inherent issues of nanoscale
contacts with electrodes in the device structures. As suchinvestigating the transport properties of heterostructured
nanowires via a noncontact method would be invaluable. In
this study, we investigate the transport properties of NiO/
MgO heterostructured nanowires by utilizing a noncontact
microwave conductivity method.
MgO/NiO core/shell heterostructured nanowires werefabricated by in situ laser molecular beam epitaxytechnique.19 The details of fabricating MgO core-nanowires
can be seen in Ref. 21. The oxygen pressure and the substrate temperature during the shell layer formations wereranged from 10−3 − 10 Pa and RT-800 ° C, respectively. Afterthe deposition, the samples were cooled down to room temperature 共RT兲 from the set temperature for 30 min. To evaluate the crystallinity of NiO/MgO heterostructured nanowire
on MgO 共100兲 substrate, x-ray diffraction measurement wasperformed. We have confirmed that NiO shell-layers wereepitaxially grown on MgO core-nanowires, as reported inRef. 19. To characterize the morphology of resultant nanowires, a field emission electron microscopy 共FESEM, HITACHI S-4300兲 at an accelerating voltage of 30 kV was used.
Prior to the FESEM observation, Pt was deposited on thesamples to prevent the charging. The microstructural properties, including the crystallinity and the MgO/NiO heterointerface, were evaluated by using a transmission electron microscopy 共TEM兲. We have confirmed 共i兲 the smooth covering
surface of MgO/NiO heterostructured nanowires by FESEM,
and 共ii兲 the well-defined core/shell interface by TEM andelectron diffraction analysis, as reported in Ref. 19. Thus,
the in situ formation technique and the crystal matching ofMgO and NiO allow us to fabricate the MgO/NiO heterostructured nanowires with well-defined heterointerfaces.19 To
characterize the transport properties of heterostructured
nanowires, a microwave conductivity measurement 共MCM兲

95, 133110-1© 2009 American Institute of Physics

Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

133110-2

Oka et al.

FIG. 1. 共Color online兲 Typical resonant curves of microwave reflected powers for MgO core-nanowires and the NiO/MgO heterostructured nanowires.
The inset shows the magnified data around the resonant frequency.

was performed. The microwave frequency and power wereset at ⬃9.1 GHz and 3 Mw, respectively, so that the motionof charge carriers cannot be disturbed by the low electricfield of the microwave. The absorbed power of microwave
by the samples was monitored as changes in the reflected
microwave from a resonant cavity 共Q = ⬃ 2500兲 with thesamples, and measured as a function of the frequency. All theabove experiments were carried out at RT. The conductivity
共⌬␴兲 of the samples is related to the reflected microwave
power at the resonant frequency 共⌬Pr / Pr兲 via the formula⌬␴ = 共1 / A兲共⌬Pr / Pr兲, where A is a sensitivity factor. The
other details of the set of apparatus were described in Ref.
22. To estimate the resistivity of nanowires from MCMs, thenanowire density and the nanowire length were analyzed byaveraging the data over 500 nanowires in FESEM images forstatistical reliability.
Figure 1 shows the typical resonant curves of microwave
reflected powers. For comparison, the data of both only MgO
core-nanowires and MgO/NiO heterostructured nanowires
were shown in the figure. The observed resonant frequency
shift for the heterostructured nanowires is due to the changeof the dielectric constant. As shown in the inset figure, the Q
factor for the heterostructured nanowires was significantly
larger than that of MgO core-nanowires. This indicates thatthe conductivity of heterostructured nanowires is muchhigher than that of MgO core-nanowires. Thus, the microwave conductivity technique is capable of detecting thetransport properties of heterostructured nanowires without
any electrodes and the related contact issues. Figure 2 shows
the resistivity of the MgO/NiO heterostructured nanowires
when varying the oxygen pressure during shell layer forma-FIG. 2. 共Color online兲 Resistivity data of the NiO/MgO heterostructured
nanowires and NiO/MgO heterothin films when varying the oxygen pressureduring NiO layer formation under 300 ° C. The data of both MCMs andFPMs were shown in the figure.

Appl. Phys. Lett. 95, 133110 共2009兲

tion under 300 ° C. For comparison, the four-probe measurement 共FPM兲 data of MgO/NiO heterothin films were shownin the figure. There are three major trends in the transport
data. First, the resistivity of both heterostructured nanowires
and heterothin films increased with increasing the oxygenpressure. In fact, this trend is completely opposite whencompared with the trend of the bulk p-type conduction
mechanism.7 Previously, we have identified that the conductivity of the low temperature grown NiO strongly correlateswith tailing the band edge via the deterioration ofcrystallinity.23 Thus, the oxidization effect on the conductivity is common in both heterostructured nanowires and thinfilms. Second, the resistivity of both heterostructured nanowires and heterothin films were nearly identical for low oxygen pressure range 10−3 Pa, indicating that the NiO quality
should be similar for both systems. Third, the discrepancy
between the heterostructured nanowires and the heterothin
films tended to be larger as the ambient oxygen pressure
tended to increase. At the highest oxygen pressure, 10 Pa, theresistivity of heterostructured nanowires was four orders ofmagnitude lower than that of heterothin films. In fact, thisdiscrepancy differs from the case of MgO/titanate heterostructured nanowires, where both heterostructured nanowires
and heterothin films showed similar resistivity trends.20
Here, we question the origin of such huge discrepancy in
MgO/NiO heterostructures. There are several possible scenarios to explain the discrepancy. First, the difference between the microwave conductivity 共ac兲 measurements andthe FP 共dc兲 measurements might cause the discrepancy via
the frequency dependence of carriers. Thus, we examined thevalidity of the MCMs by comparing with the FPM data for
MgO/NiO heterothin films grown under 300 ° C and 10−3
− 10 Pa. The data are shown in Fig. 2. Clearly both data arecomparable, and at least the frequency dependence alone
cannot explain the observed discrepancy in four orders ofmagnitude. In fact, this agreement is theoretically understandable when using relatively low mobility materials including oxides, since the hole mobility of single crystalline
NiO at RT was reported to be 35 cm2 V−1 s−1.24 Second possible model assumes the drastic degradation of crystallinity
for entire NiO shell layers of heterostructured nanowires incomparison with the heterothin films. In this scenario, the“entire” NiO shell layers must be strongly deteriorated. Although our TEM analysis could not identify such crystal degradations, this scenario cannot be excluded by considering
the degree of the crystal degradations and the effect on thetransport properties. Third model is based on the large
specific surface area effect on heterostructured nanowires
compared with heterothin films. The specific surface area ofheterostructured nanowires can be estimated to be approximately four orders of magnitude larger than that of heterothin films. If the crystal disorders naturally occurred atsurfaces and/or interfaces introduce mobile carriers,25 thethird model can explain the discrepancy between the heterostructured nanowires and the heterothin films on the transport. It is noted that the major difference between the second
model and the third model is either “bulk” effect or “surfaceand interface” effect. In the third model, the sheet resistanceshould be similar in both heterostructured nanowires and theheterothin films. The calculated sheet resistances were shownin Table I with the comparison to the resistivity data. Thesheet resistances are 1.60⫻ 1010 共⍀ sq−1兲 for heterostruc-Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

133110-3

Appl. Phys. Lett. 95, 133110 共2009兲

Oka et al.

TABLE I. Comparison between MgO/NiO heterostructured nanowires and
MgO/NiO heterothin films on the resistivity and the sheet resistance data.

MgO/NiO heterostructured nanowires
MgO/NiO heterothin films

Resistivity
共⍀ cm兲

Sheet resistance共⍀ sq−1兲

3.95⫻ 1039.37⫻ 1061.60⫻ 1010
0.94⫻ 1010

tured nanowires, 0.94⫻ 1010 共⍀ sq−1兲 for heterothin films,
whereas the resistivity data are 3.95⫻ 103 共⍀ cm兲 for heterostructured nanowires, 9.37⫻ 106 共⍀ cm兲 for heterothin films.
Clearly both values of the sheet resistances are comparable,
supporting the third model. In addition, we further investigated the thickness dependence of NiO shell layers on theresistivity. If the second model based on the “bulk” effectdominates the transport of heterostructured nanowires, themeasured resistivity should be insensitive to the variation of
the shell layer thickness. On the other hand, the sheet resistance should be constant even increasing the shell layer
thickness if the third model based on the “surface and interface” effect can explain the transport of heterostructured
nanowires. Figure 3 shows the thickness dependences on theresistivity and the sheet resistance. It can be clearly seen thatthe resistivity increased with increasing the shell layer thickness, which is in fact inconsistent with the second model. On
the other hand, the calculated sheet resistances were almost
constant independent of the shell layer thickness. Thus, theseresults highlight that the discrepancy between the heterostructured nanowires and the heterothin films on the resistivity in Fig. 2 can be interpreted in terms of the difference ofthe specific surface area and the surface/interface transport
events. Further remained issue is that the major electricalconduction occurs at either “surface” or “interface.” Although we do not have direct experimental evidences to address this issue, there are two indirect implications to support
the transport events at the “surface” rather than at “interface.” First, TEM analysis demonstrated the absence of any
observable crystal imperfections at the heterointerface.19
Second, the agreement between the heterostructured nanowires and the heterothin films on the sheet resistances supports the surface effects rather than the heterointerface effects, because in the FPMs the measurable resistance shouldreflect the transport events mainly at the film surface rather
than that at the heterointerface between the film and the substrate.
In summary, we have investigated the transport properties of NiO/MgO heterostructured nanowires by utilizing thenoncontact microwave conductivity method. We found theFIG. 3. 共Color online兲 Thickness dependences on the resistivity and thesheet resistance for NiO/MgO heterostructured nanowires.

substantial discrepancy in four orders of magnitude between
the NiO heterostructured nanowires and NiO heterothin films
on the resistivity when the bulk resistivity tended to increase,
whereas such discrepancy was no longer observable for relatively conductive region. The discrepancy between the heterostructured nanowires and thin films on the transport properties can be interpreted in terms of both the large specificsurface area of nanowire structures and the surface transport
events of insulative NiO. Thus, noncontact microwave transport measurements, performed for at least 1010 nanowires inthis study, offer statistically reliable transport data of heterostructured nanowires, and would pursue our understanding as
to the nanoscale transport events of nanowires by comparing
with contact transport measurements of single nanowire.
The authors wish to thank M. Kanai for constructive
advice and T. Ishibashi for his invaluable technical support.
This work is partially funded by SCOPE.
K. Xu and J. R. Heath, Nano Lett. 8, 136 共2008兲.
T. Yanagida, T. Kanki, B. Vilquin, H. Tanaka, and T. Kawai, Phys. Rev. B
70, 184437 共2004兲.
3
W. Eerenstein, M. Wiora, J. L. Prieto, J. F. Scott, and N. D. Mathur,Nature Mater. 6, 348 共2007兲.
4
R. Waser and M. Aono, Nature Mater. 6, 833 共2007兲.
5
K. Nagashima, T. Yanagida, H. Tanaka, and T. Kawai, J. Appl. Phys. 101,026103 共2007兲.
6
C. M. Lieber and Z. L. Wang, MRS Bull. 32, 99 共2007兲.
7
R. Newman and R. M. Chrenko, Phys. Rev. 114, 1507 共1959兲.
8
J. McCord, R. Kaltofen, T. Gemming, R. Hühne, and L. Schultz, Phys.
Rev. B 75, 134418 共2007兲.
9A. K. M. S. Chowdhury, S. A. Akbar, S. Kapileshwar, and J. R. Schorr,J. Electrochem. Soc. 148, G91 共2001兲.
10
H. Kamal, E. K. Elmaghraby, S. A. Ali, and K. Abdel-Hady, Thin Solid
Films 483, 330 共2005兲.
11K. Kinoshita, T. Tamura, M. Aoki, Y. Sugiyama, and H. Tanaka, Appl.
Phys. Lett. 89, 103509 共2006兲.
12
I. K. Yoo, B. S. Kang, Y. D. Park, M. J. Lee, and Y. Park, Appl. Phys. Lett.
92, 202112 共2008兲.
13
J. Y. Son and Y.-H. Shin, Appl. Phys. Lett. 92, 222106 共2008兲.
14
Y. Lin, T. Xie, B. Cheng, B. Geng, and L. Zhang, Chem. Phys. Lett. 380,521 共2003兲.
15
S. I. Kim, J. H. Lee, Y. W. Chang, S. S. Hwang, and K. H. Yoo, Appl.
Phys. Lett. 93, 033503 共2008兲.
16
S. Han, C. Li, Z. Liu, B. Lei, D. Zhang, W. Jin, X. Liu, T. Tang, and C.
Zhou, Nano Lett. 4, 1241 共2004兲.
17
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.
18
C. Li, B. Lei, Z. Luo, S. Han, Z. Liu, D. Zhang, and C. Zhou, Adv. Mater.
17, 1548 共2005兲.
19
K. Oka, T. Yanagida, K. Nagashima, H. Tanaka, and T. Kawai, J. Am.
Chem. Soc. 131, 3434 共2009兲.
20
K. Nagashima, T. Yanagida, H. Tanaka, S. Seki, A. Saeki, S. Tagawa, andT. Kawai, J. Am. Chem. Soc. 130, 5378 共2008兲; A. Marcu, T. Yanagida,K. Nagashima, K. Oka, H. Tanaka, and T. Kawai, Appl. Phys. Lett. 92,173119 共2008兲.
21K. Nagashima, T. Yanagida, H. Tanaka, and T. Kawai, Appl. Phys. Lett.
90, 233103 共2007兲; J. Appl. Phys. 101, 124304 共2007兲; A. Marcu, T.
Yanagida, K. Nagashima, H. Tanaka, and T. Kawai, ibid. 102, 016102
共2007兲; T. Yanagida, K. Nagashima, H. Tanaka, and T. Kawai, Appl. Phys.
Lett. 91, 061502 共2007兲; T. Yanagida, K. Nagashima, H. Tanaka, and T.
Kawai, J. Appl. Phys. 104, 016101 共2008兲; K. Nagashima, T. Yanagida, K.
Oka, H. Tanaka, and T. Kawai, Appl. Phys. Lett. 93, 153103 共2008兲; T.
Yanagida, A. Marcu, H. Matsui, K. Nagashima, K. Oka, K. Yokota, M.
Taniguchi, and T. Kawai, J. Phys. Chem. C 112, 18923 共2008兲.
22
Y. Yamamoto, T. Fukushima, Y. Suna, N. Ishii, A. Saeki, S. Seki, S.
Tagawa, M. Taniguchi, T. Kawai, and T. Aida, Science 314, 1761 共2006兲.
23
K. Oka, T. Yanagida, K. Nagashima, H. Tanaka, and T. Kawai, J. Appl.
Phys. 104, 013711 共2008兲.
24W. E. Spear and D. S. Tannhauser, Phys. Rev. B 7, 831 共1973兲.
25
C. H. Olk, S. M. Yalisove, and G. L. Doll, Phys. Rev. B 52, 1692 共1995兲.
12Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp