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Nonvolatile Bipolar Resistive Memory Switching inSingle Crystalline NiO Heterostructured Nanowires
Keisuke Oka, Takeshi Yanagida, Kazuki Nagashima, Hidekazu Tanaka, and Tomoji Kawai
J. Am. Chem. Soc., 2009, 131 (10), 3434-3435• DOI: 10.1021/ja8089922 • Publication Date (Web): 19 February 2009
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Nonvolatile Bipolar Resistive Memory Switching in Single Crystalline NiOHeterostructured Nanowires
Keisuke Oka, Takeshi Yanagida,* Kazuki Nagashima, Hidekazu Tanaka, and Tomoji Kawai*
Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka Ibaraki, Osaka, 567-0047, Japan,and PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan
Received November 24, 2008; E-mail: yanagi32@sanken.osaka-u.ac.jp; kawai@sanken.osaka-u.ac.jp

Nickel oxide (NiO) has recently attracted much attention due tothe resistive memory switching phenomena based on the electricallystimulated change of the resistance of a metal-insulator-metal(MIM) memory cell, frequently called resistive switching RAM(ReRAM).1,2 The NiO ReRAM devices are expected to be future
nonvolatile memory devices used as alternatives to the current flash
memory technology.1 The nonvolatile memory characteristics ofNiO have been intensively investigated using the thin film forms,
and excellent memory characteristics have been demonstrated.3-15
However, to achieve high-density memory and improve theperformance of the devices, it is crucial to reduce the size of theNiO cells beyond the limitation of current lithographic length scales.
In addition, details of NiO resistive memory switching mechanisms,
including the switching types (bipolar and/or unipolar), have notbeen well understood. Thus the scaling down of the NiO structures
is strongly desired not only for understanding the underlying
memory mechanisms within a confined nanoscale but also for
improving the device characteristics of ReRAM.14 The bottom-up
approach using self-assembled nanowires is a promising solution
for scaling down the size of memory devices. Thus, single
crystalline NiO nanowires formed via a self-assembling fashion
would be a candidate for overcoming the above issues. Howeverdifficulties in both fabricating well-defined single crystalline NiO
nanowires and evaluating electrically the insulative NiO nanowires(band gap, nearly 4 eV) have impeded investigation of the resistive
memory switching events in single crystalline NiO nanowires. Here
we demonstrate for the first time the nonvolatile bipolar resistive
memory switching in single crystalline NiO heterostructured
nanowires.
Single crystalline NiO heterostructured nanowires were synthesized by an in situ heterostructured nanowire formation techniqueusing laser MBE.18 The NiO shell layers were deposited onto single
crystalline MgO nanowires by ablating the NiO pellet target(99.99%, Kojundo Chemical). The MgO nanowires were fabricatedby the metal catalyst-mediated vapor-liquid-solid (VLS) mechanism, and details of MgO nanowire formations can be seenelsewhere.16 Note that the NiO shell layer formations onto the MgOnanowires were performed without atmospheric exposures. Figure
1 shows the field emission scanning electron microscopy (FESEM)
and the high-resolution transmission electron microscopy (HRTEM)
images of fabricated NiO heterostructured nanowires. The NiO shelllayer was formed under 800 °C and 10 Pa of oxygen pressure. TheFESEM image (Figure 1a) reveals the macroscopic uniformity ofNiO heterostructured nanowires without any agglomerations of NiOshell layers. The microscopic uniformity of NiO heterostructured
nanowires can be seen in the HRTEM images (Figure 1b). The
well-defined heterointerface was maintained for samples prepared
above 300 °C. The single crystalline nature of NiO heterostructured
nanowires was further confirmed by a magnified image (inset ofFigure 1b). The XRD measurements for NiO heterostructured3434 9 J. AM. CHEM. SOC. 2009, 131, 3434–3435

nanowires also confirmed the epitaxy growth of single crystalline
NiO shell layers (Supporting Information). In addition, the inverse
FFT analysis for the HRTEM image clarified the absence of any
observable crystal dislocations at the heterointerface, revealing thewell-defined heterointerface of the single crystalline NiO heterostructured nanowires. The size of NiO heterostructured nanowires
can be easily controlled by varying the MgO core-nanowire
diameter16 and changing the NiO shell thickness in the unit cell. It
should be emphasized that the ideal crystal combination between
NiO and MgO, including the same crystal structures (rock-salt) andthe small lattice mismatch (0.7%), and the use of the in situ
heterointerface formation technique are essential to fabricate thewell-defined single crystalline NiO heterostructured nanowires.
The resistive memory switching behavior of single crystalline
NiO heterostructured nanowires was investigated by electricalmeasurements using a conductive-atomic force microscopy (CAFM) method.17 The electrical properties were investigated bymeasuring the current (I)-voltage (V) curve of a single crystalline
NiO heterostructured single nanowire at room temperature. The
measurement structures were fabricated by transferring synthesized
NiO heterostructured nanowires onto an oxidized Si substrate, andthen a Pt electrode with a sharp edge was deposited onto thesubstrate.17 After confirming the exact location of the nanowires
on the substrate using a nondestructive tapping mode, the C-AFM
measurements were performed at the nanowire position ∼1000 nm
away from the Pt electrode using a conductive Ir/Pt coated Sicantilever (details in Supporting Information). Figure 2 shows thetypical I-V curve of a NiO heterostructured nanowire with a 30nm diameter (10 nm, MgO core-nanowire diameter; 10 nm, NiOshell thickness). The cross-sectional area of NiO shell layers forresistive switching is ∼10-3 µm2, which is in fact at least 5 ordersof magnitude smaller than the values of previously reported NiO
thin film devices, typically 102 µm2.3-15 In the I-V measurement,
initially the NiO nanowires were very insulative and the resistivity
was over 1011 Ω. When applying the negative electric field, thecurrent through the NiO nanowire increased drastically up to 10-8
A from the 10-11 A range, indicating the occurrence of a “SET”process. After the SET process, the low resistance of a NiO
nanowire was kept constant as long as the electric field wasnegative. On the other hand, when a positive electric field wasapplied, the current through the NiO nanowire decreased drasticallydown to 10-11 A, indicating the occurrence of a “RESET” process.
Thus, we demonstrate for the first time that the single crystalline
NiO heterostructured nanowires exhibit a bipolar resistive memory
switching. Note that the electric fields for SET and RESET werecomparable with previous data using NiO thin film devices withbipolar switching.15 The repeatability of the SET and RESET
processes was confirmed over several cycles (Supporting Information). We further confirmed the occurrence of the resistive memory
switching within the nanowires by performing the measurements
10.1021/ja8089922 CCC: $40.75  2009 American Chemical Society

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In conclusion, we have demonstrated nonvolatile bipolar resistive
memory switching in single crystalline NiO heterostructured
nanowires for the first time. The self-assembled NiO nanowires
are expected to open up opportunities to explore not only thedetailed nanoscale mechanisms in NiO resistive memory switching
but also next-generation nanoscale nonvolatile memory devices withthe potential for high-density device integration and improved
memory characteristics.
Acknowledgment. This work is partially funded by SCOPE.
The authors thank Dr. Kanai, Dr. Takagi, and Prof. Matsumoto for
their stimulating discussions and Mr. Ishibashi for his technicalsupports during this work.

Figure 1. (a) FESEM image of NiO heterostructured nanowires grown on
single crystal MgO substrate; (b) HRTEM image of single crystalline NiO
heterostructured nanowires. Inset figure shows the magnified image.

Supporting Information Available: Detailed explanations as to theevaluations of NiO heterostructured nanowires and the conductive AFM
measurements. This material is available free of charge via the Internetat http://pubs.acs.org.
References

Figure 2. I-V characteristics of single crystalline NiO heterostructured
nanowires using C-AFM method.

on more conductive NiO heterostructured nanowires obtained byvarying the shell layer formation.18 As the conductivity of NiO
nanowires increased, I by C-AFM increased. There are several
interesting implications from these experimental results whencompared with previous works as to NiO ReRAM thin film
devices.15 Single crystalline NiO heterostructured nanowires down
to several tens of nanometers exhibit bipolar resistive memory
switching, not unipolar memory switching. In the bipolar switching,
the polarity is closely related to the switching mechanism including
a charged crystal vacancy and others, while the unipolar switching
can occur without the polarity change. Thus the bipolar switching
behavior of single crystalline NiO nanowires essentially differs fromthe unipolar switching behavior of polycrystalline NiO thin films.
Although only a few reports exist of NiO epitaxial thin film devices
showing the bipolar switching,15 which is similar to our results
using single crystalline NiO nanowires, the switching type for thesingle crystalline NiO nanostructures had not been evident.19 Thus
our experimental results infer that the bipolar switching mechanism
governs the NiO ReRAM effect on the nanoscale. Although thecomplex switching mechanisms in NiO need to be further investigated, these experimental findings would be invaluable not onlyfor understanding the detailed switching mechanisms of NiO on aconfined nanoscale but also for developing nanoscale memory
devices.

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JA8089922

J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009 3435