Research Article
www.acsami.org

Tailoring Nucleation at Two Interfaces Enables Single Crystalline NiO
Nanowires via Vapor−Liquid−Solid Route
Kazuki Nagashima,† Hideto Yoshida,‡ Annop Klamchuen,§ Masaki Kanai,† Gang Meng,† Fuwei Zhuge,†
Yong He,† Hiroshi Anzai,† Zetao Zhu,† Masaru Suzuki,† Mickael̈ Boudot,† Seiji Takeda,‡
and Takeshi Yanagida*,††The Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, JapanThe Institute of Scientiﬁc and Industrial Research, Osaka University, 8-1 Mihogaoka Ibaraki, Osaka 567-0047, Japan§
National Nanotechnology Center (NANOTEC), NSTDA, 111 Thailand Science Park, Pathumthani 12120, Thailand‡

ABSTRACT: Here we show a rational strategy to fabricate single crystalline NiO
nanowires via a vapor−liquid−solid (VLS) route, which essentially allows us to
tailor the diameter and the spatial position. Our strategy is based on the
suppression of the nucleation at vapor−solid (VS) interface, which promotes
nucleation only at the liquid−solid (LS) interface. Manipulating both the suppliedmaterial ﬂuxes (oxygen and metal) and the growth temperature enables
enhancement of the nucleation only at the LS interface. Furthermore, this strategy
allows us to reduce the growth temperature of single crystalline NiO nanowires
down to 550 °C, which is the lowest growth temperature so far reported.

KEYWORDS: nickel oxide nanowire, vapor−solid−liquid process, vapor ﬂux control, single crystalline oxide nanowire,
memristive switching
Pa; In, 2.82 × 10−1 Pa; Sn, 4.14 × 10−4 Pa at 850 °C).40,41 This
low vapor pressure of Ni might hinder the nanowire growth of
NiO via VLS route at conventional growth temperatures (up to
800 °C) because the nucleation at the vapor−solid (VS)interface must be enhanced for such low vapor pressurematerials, which is detrimental for VLS process. The
straightforward approach to overcome this issue might be theuse of a higher growth temperature (for example higher than
1000 °C) to suppress the nucleation at the VS interface andalso enhance the nucleation only at the LS interface for the VLS
process.27,28 However, such a high temperature processstrongly limits the application range due to the integrations
with other processes and materials. Therefore, a realization of
the VLS process for NiO at a relatively low temperature range
is desired. Here, we propose a rational strategy to fabricate asingle crystalline NiO nanowire via the VLS route by tailoring a
nucleation at both VS and LS interfaces. This strategy is basedon the so-called “material ﬂux window” principle. 42,43
Manipulating both the supplied material ﬂuxes (oxygen andmetal) and the growth temperature enables us to enhance thenucleation only at the LS interface. Furthermore, this strategy
allows us to reduce the growth temperature of single crystalline

1. INTRODUCTION
Nickel oxide (NiO), which is a simple binary metal oxide withthe rock-salt crystal structure, is an attractive functional material
due to its fascinating fundamental physical properties, such as
antiferromagnetism, p-type semiconductivity, catalytic properties, and others,1−4 and also their diverse device applications,
including the use as a supercapacitor,5,6 nonvolatile memory,7−10 lithium ion battery,11,12 and solar cell.13,14 Among
various nanostructures of NiO, single crystalline NiO nanowires
are promising because of the applicability to fundamentally
study nanoscale physical properties of NiO15 and also NiObased functional nanodevices by utilizing the well-deﬁned andtailorable surface properties with the large area.16,17 Although
there are various methods to form NiO nanowires,18−31 the socalled vapor−liquid−solid (VLS) process with metal catalysts
oﬀers a promising way to fabricate single crystalline NiO
nanowires because the VLS process allows us to tailor the
diameter, the spatial position, and the heterostructures ofnanowires, which are not attainable by other nanowire growthmethods.32−39 However, the VLS nanowire growth of NiO has
been substantially limited and diﬃcult due to the lack ofunderstanding of the fundamental process of VLS growth for
NiO. One of possible reasons for this diﬃculty might be therelatively low vapor pressure of Ni (5.97 × 10−8 Pa at 850 °C),
which is substantially lower (at least 4 orders up to 12 orders)than those of metals, whose oxide nanowires are easily formed
via VLS route (for example, Zn, 6.12 × 104 Pa; Mg, 8.87 × 103© 2016 American Chemical Society

Received: August 4, 2016
Accepted: September 27, 2016
Published: September 27, 2016
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Figure 1. (a) FESEM images of NiO nanowires grown onto a MgO(100) single crystalline substrate. The growth temperature, the Ni ﬂux, and theoxygen partial pressure were 850 °C, 1.8 × 1018 atoms/cm2 s, and 10−3 Pa, respectively. The left part shows a top view image, and the right partshows a cross-sectional image. (b) TEM images of NiO nanowires. The left part shows an overview image of a NiO nanowire. Inset in the left part isa magniﬁed image around the catalyst at the tip of NiO nanowire. The right part shows a high resolution TEM image of a NiO nanowire. Inset in theright part is an SAED pattern, indicating the single crystal nature and cubic structure of NiO nanowire. (c) EDS spectra of a NiO nanowire taken atnear the catalyst (yellow) and the nanowire body (green). (d) XRD pattern of NiO nanowires on MgO substrate. (e) Spatial selectivity of present
NiO nanowires.

NiO nanowires via the VLS route down to 550 °C, which is thelowest growth temperature so far reported.27,28

emission scanning electron microscopy (FESEM, JEOL JEM-7001F),
and the 300 kV transmission electron microscopy (TEM, JEOL JEM3000F) equipped with energy dispersive X-ray spectroscopy (EDS),
and the 200 kV scanning transmission electron microscopy (STEM,JEOL JEM-ARM200F), were utilized to evaluate the crystal structure,
the morphology, and the composition of fabricated oxide nanowires.
The electrical transport measurement of a NiO nanowire wasconducted in the form of a single oxide nanowire device, which wasconstructed onto the SiO2/Si substrate by the 30 kV electron beamlithography technique.44 For the electrical contact, a Pt electrode was
sputtered via the RF magnetron sputtering at 50 W in Ar 1 Pa. The
electrical transport measurement was carried out at room temperature
in atmospheric conditions.

2. EXPERIMENTAL SECTION
For the NiO nanowire growth, we utilized the pulsed laser deposition(PLD) technique using an ArF excimer laser (Lamda-Physik,
COMPex 102, λ = 193 nm).32−35 Prior to the nanowire growth, ∼1
nm thick Au, which was used as catalyst for nanowire growth, wasdeposited onto a MgO (100) single crystal substrate by dc sputtering(SANYU Electron, SC701-HMC). The background pressure was ca. 1
× 10−5 Pa. Then, the O2/Ar mixture gas was introduced into thechamber with varying oxygen partial pressures from 10−5 to 10 Pa by
maintaining 10 Pa of the total pressure. The substrate temperature wasraised up to the objective value within 20 min before the deposition.
The vapor source was supplied by ablating the NiO tablet (99.9%
pure). To control the vapor ﬂux, the laser energy and the substrate−tablet distance were varied in the ranges 40−80 mJ and 30−45 mm,
respectively. In addition, the attenuator was introduced to further
control the vapor ﬂux at a small ﬂux range. The Ni ﬂux with units of
atom/cm2 s was evaluated by measuring the volume of NiO thin ﬁlms
deposited at room temperature since the re-evaporation rate is
negligible at that temperature condition. The other experimental
conditions such as the repetition rate, the growth time, and the coolingtime after the nanowire growth were set to be 10 Hz and 60 and 30
min, respectively. The X-ray diﬀraction (XRD, Philips), the 30 kV ﬁeld

3. RESULTS AND DISCUSSIONFirst, we characterize the fabricated NiO nanowires, and thendiscuss the microscopic VLS process of NiO nanowires interms of “material ﬂux window” concept.42 Figure 1a shows the
typical FESEM images of fabricated NiO nanowires grown on a
MgO(100) single crystal substrate. The growth temperature, Ni
metal ﬂux, and oxygen partial pressure were 850 °C, 1.8 × 1018
atoms/cm2 s, and 10−3 Pa, respectively. The fabricated NiOnanowires grow perpendicular to the substrate due to the smalllattice mismatch between MgO and NiO (∼0.95%), similar to
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Figure 2. (a) SEM images of NiO nanowires when varying the vapor ﬂux (oxygen partial pressure 10−3 Pa−1 Pa). (b) Nanowire density mapping asfunctions of both Ni ﬂux and oxygen partial pressure, indicating the critical role of the vapor ﬂux on the VLS growth of oxide nanowires. (c) The
growth rate of NiO nanowires (VLS growth) and NiO ﬁlms (VS growth) as functions of oxygen partial pressure. The Ni ﬂux for part c was 5 × 1017
atoms/cm2 s. (d) VLS nanowire growth rate of NiO nanowires as a function of Ni ﬂux for various oxygen partial pressure conditions. The error bars
show the standard deviation. Parts c and d show the ﬁtted lines for experimental data.

when we strictly controlled the vapor ﬂuxes (metal andoxygen), as explained in the following discussion.
The importance of vapor ﬂux for a VLS nanowire growth
process has been proposed by using molecular dynamics
simulation, which suggests the presence of a so-called “material
ﬂux window” where the nucleation preferentially occurs only at
the liquid−solid (LS) interface.42,47,48 The presence of such amaterial ﬂux window was experimentally validated for the VLS
process for metal oxide nanowires.42,43 Within the frameworkof this concept, the size of the critical nucleus at the LSinterface is smaller than that at the vapor−solid (VS) interface
due to the surface energy gain by the surrounding liquid
catalyst atoms, resulting in the preferential crystal growth onlyat the LS interface. Since the nucleation rate of the critical
nucleus depends on the partial pressure of vapor, i.e., vapor ﬂux,
controlling the vapor ﬂux is crucial for the VLS nanowire
growth process.42,43,49−51 Since both metal and oxygen arecomponents of metal oxides, it is interesting to investigate theeﬀect of each vapor ﬂux, including Ni metal and oxygen ﬂuxes,on the VLS process. By utilizing the present PLD deposition
technique, it is possible to control individually the Ni metal ﬂux
and oxygen ﬂux with other constant experimental conditions byaltering the laser power and the target-to-substrate distance forthe ablation of the Ni metal target and the oxygen partialpressure in the vacuum chamber.
Figure 2a shows the typical SEM images of fabricated NiOnanowires when varying the vapor ﬂux (oxygen). The Ni metal
ﬂux was kept at 1.8 × 1018 atoms/cm2 s, and the substratetemperature was 850 °C. As seen in the ﬁgure, the vapor ﬂux
strongly aﬀects the shape of fabricated NiO nanowires. With anincrease in the vapor ﬂux, the NiO nanowire shape tends to betapered and the VS ﬁlm growth tends to be dominant rather
than the VLS nanowire growth. For the highest vapor ﬂux, thenanowire shapes are no longer observable, and only ﬁlm

NiO epitaxial ﬁlms on MgO(100) substrate.45 Figure 1b shows
the TEM images of a NiO nanowire. The nanowire diameters
ranged from 20 to 70 nm. The preferential [100] growth
direction, the single crystallinity, and the cubic structure (lattice
constant 4.17 Å) of the NiO nanowires were conﬁrmed by thehigh resolution TEM image and the selective area electron
diﬀraction (SAED) pattern. As seen in Figure 1b, the sphericalshaped catalyst exists at the tip of NiO nanowire. As shown in
Figure 1c, the EDS spectra data measured near the catalyst andat the nanowire body reveal that the catalyst and the nanowire
are mainly composed of Au and NiO, respectively. The
presence of Au catalyst at the tip of the nanowires suggests theoccurrence of the VLS process for the present NiO
nanowires.46 Figure 1d shows the XRD pattern of NiO
nanowires. XRD data only shows the NiO(002) related peaks,
and any other peaks are not observed, which is consistent withthe results of TEM-SAED analysis. To further conﬁrm theoccurrence of the VLS process for the present NiO nanowires,
we performed the spatial patterning of Au catalysts on thesubstrate because NiO nanowires should grow only at the
spatial positions where the Au catalysts are patterned on thebasis of the VLS process. Figure 1e shows the SEM images offabricated NiO nanowires when the Au catalysts are spatiallypatterned. As can be seen, the fabricated NiO nanowires grew
only where the Au catalysts were patterned. Note that thescaling limitation, the density, and the size of the present spatial
patterning depend on the spatial resolution of e-beamlithography, which is around 50 nm in our system, although
better spatial resolution can be achievable by using a moreadvanced e-beam lithography system. Thus, these results
highlight that the fabricated nanowires are single crystalline
NiO via the VLS process with Au catalysts. It should be notedthat these successful NiO nanowires could be fabricated only
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the diﬀerence between the vapor pressure and the saturated
vapor pressure of target materials. The important point here isthat the saturated vapor pressure shows strong temperaturedependence, as shown in Figure 3. In general, the lower the

structures are formed. To more quantitatively study the eﬀectof vapor ﬂuxes, we measured the nanowire density values fromthe SEM data. Figure 2b shows the NiO nanowire density datawhen varying Ni metal ﬂux and oxygen ﬂux (oxygen partialpressure). The NiO nanowires were grown at the constanttemperature (850 °C). As can be seen, NiO nanowires can begrown within a rather limited ﬂux range. For example, NiO
nanowires were grown precisely only within the Ni metal ﬂuxes,
ranging from 5 × 1017 to 1.8 × 1018 atoms/cm2 s in theconstant oxygen partial pressure of 10−3 Pa. Thus, the VLS
process of NiO nanowires requires the strict control of vapor
ﬂuxes including both Ni metal ﬂux and oxygen partial pressures.
This trend is consistent with the concept based on the “material
ﬂux window” of VLS process.42
To further examine the eﬀect of vapor ﬂuxes, we measured
the growth rate of NiO nanowires from the cross-sectional
SEM data. Figure 2c shows the growth rate of NiO nanowires(LS growth) and NiO ﬁlms (VS growth) as a function ofoxygen partial pressure. The NiO nanowires were grown at 850°C with the constant Ni metal ﬂux of 5 × 1017 atoms/cm2 s. As
can be seen, the growth rate of NiO nanowires via the VLS
process showed the maximum at the oxygen partial pressure of
10−4 Pa. On the other hand, the growth rate of VS ﬁlms
monotonically increased with an increase in the oxygen partialpressure nearly above the oxygen partial pressure of 10−4 Pa, at
which the growth rate of NiO nanowires showed the maximum.
Clearly, the occurrence of the VS ﬁlm growth hinders the VLS
process of nanowires with variation in the oxygen partialpressure. Figure 2d shows the growth rate of NiO nanowires
when varying only the Ni metal ﬂux. The NiO nanowires weregrown at constant temperature (850 °C), and various oxygen
partial pressures ranged from 10−3 to 1 Pa. For the oxygen
partial pressures of 10−2 and 10−1 Pa, the growth rate of NiO
nanowires showed the maximum at a certain Ni metal ﬂux (ca.
1.8 × 1018 atoms/cm2 s for 10−2 Pa, ca. 1.1 × 1018 atoms/cm2 s
for 10−1 Pa). In the case of the oxygen partial pressure of 1 Pa,
any nanowire structures were not observable, and only ﬁlm
structures appeared even with varying the Ni metal ﬂux. Similar
to the experimental trend upon varying the oxygen partialpressure (Figure 2c), the VLS nanowire growth rate tends todecrease when the VS ﬁlm growth tends to be dominant whenincreasing the Ni metal ﬂux. Thus, these results highlight thatprecisely manipulating the vapor ﬂuxes including Ni metal andoxygen ﬂuxes is essential to fabricate NiO nanowires via the
VLS process. In other words, tailoring the nucleation at two
interfaces (LS and VS interfaces) via controlling the vapor ﬂux
is a key process to accomplish the VLS process of NiO
nanowires.
On the basis of the above implications, we further extend theconcept to reduce the growth temperature of the VLS process
for NiO nanowires since the relatively high growth temperature
for the VLS process has limited the application range. Within
the framework of the VLS process, the growth temperaturemust be high enough for catalysts to be in a liquid state.
However, recent studies reveal that there is another growth
process, the so-called vapor−solid−solid (VSS), when thegrowth temperature tends to be low.52 In addition, the growthtemperature should be also high enough for the nucleation at
an LS interface but not for the nucleation at a VS interface. On
the basis of the above concept for material vapor ﬂuxes, it might
be possible to reduce the growth temperature of the VLS
process via controlling the vapor ﬂuxes, since the nucleation
rate at the interface is determined by supersaturation, deﬁned as

Figure 3. Thermodynamic phase diagram for the Ni nucleation at theLS and VS interface. When the vapor pressure is lower than that of thenucleation limit in catalyst, all Ni vapor is re-evaporated, and no
nucleation occurs both at LS and VS interfaces. On the other hand,when the vapor pressure is higher than the saturated vapor pressure,the nucleation occurs both at LS and VS interfaces. Controlling thevapor pressure higher than the nucleation limit and lower than thesaturated vapor pressure promotes the nucleation only at the LSinterface, resulting in nanowires. When the temperature is reduced,
appropriate vapor pressure for nanowire growth is varied. Therefore,
precise control of vapor pressure is crucial for fabricating the nanowire
at lower temperature.

temperature is, the lower the saturated vapor pressure is.41 As
illustrated in Figure 3, it should be possible to reduce thegrowth temperature of the VLS process by decreasing the
supplied vapor ﬂuxes.
Figure 4a shows the FESEM images of NiO nanowires withvarying the growth temperature. In this experiment, the Ni ﬂux
was controlled to be as low as 7 × 1017 atoms/cm2 s, and theoxygen partial pressure was ﬁxed to be 10−3 Pa. Note that theNiO nanowire growth was not observable below 800 °C under
the Ni ﬂux condition of 2.3 × 1018 atoms/cm2 s. Clearly,
nanowire growth was observable even at 550 °C, which is much
lower than any growth temperatures so far reported.27,28 Thus,
we successfully reduced the growth temperature of the VLS
process for NiO nanowires by controlling the vapor ﬂux basedon the proposed concept. Figure 4b shows the temperaturedependence on the growth rate of NiO nanowires. Below 700°C, the growth rate increases with increasing the growthtemperature, and above 700 °C the growth rates decreases. The
trend below 700 °C can be understood in terms of the changeof nucleation rate for the VS growth accompanied with thechange of the saturated vapor pressure. In fact, the size of NiO
nanowires tended to be larger at the lower growth temperature,indicating the increase of VS growth at the sidewall of NiO
nanowires. However, the trend of the growth rate over 700 °C
was completely opposite to the aforementioned situation. We
found that this behavior can be understood by taking into
account the competitive collection of supplied vapor species
between adjacent nanowires. When we increased the growthtemperature over 700 °C, the nanowire density greatly
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Figure 4. (a) FESEM images of NiO nanowires grown at various temperature conditions. The Ni ﬂux and the oxygen partial pressure were 7 × 1017
atoms/cm2 s and 10−3 Pa, respectively. (b) Temperature dependence on the growth rate and the density of NiO nanowires. The error bars show thestandard deviation.

Figure 5. Cross-sectional TEM images of NiO nanowires grown at 620 °C: (a) overview, (b) magniﬁed single nanowire, (c) SAED pattern, and (d)high resolution image. These data indicate the single crystal nature of NiO nanowires.

nanowires, we performed TEM observations. Figure 5 shows
the cross-sectional TEM images of NiO nanowires grown at
620 °C. Although there are slight tapering and the VS ﬁlm
underneath the NiO nanowires, the single crystalline nature ofNiO nanowires was maintained as clearly seen in the magniﬁed
TEM image and the SAED pattern.
Finally, we show the device application of fabricated single
crystalline NiO nanowires. Figure 6a shows the FESEM imageof a NiO nanowire device in contact with Pt electrodes. The
device was composed of a single NiO nanowire. The gap size
between the Pt electrodes was 200 nm. Figure 6b shows the
typical current−voltage (I−V) characteristics of the NiO
nanowire device. The current compliance of 10−9 A wasapplied to prevent an irreversible electrical breakdown of thedevice. The measurement was carried out at room temperature
in atmospheric conditions. As seen in the I−V curve, the device
clearly showed a nonvolatile memory switching.7−10 Note thatthe operation current of NiO nanowire device (<10−9 A) was
extremely small compared with that of conventional NiO ﬁlm
devices (∼10−4−10−2 A).7,54,55 This is interpreted in terms ofthe conduction within the small cross-sectional area of NiO
nanowire device (ca. 2 × 103 nm2), and this interesting feature
is beneﬁcial for low power consumption of the NiO nanowirebased electronic device applications. Here we analyze theelectrical transport of NiO nanowires during the nonvolatilememory switching. Figure 6c shows the ON state resistances asa function of current density. For this measurement, thecompliance currents were varied from 1 × 10−10 to 5 × 10−8 A.

increased as shown in Figure 4b; i.e., the internanowire distancedecreased. The decrease of internanowire distance causes thecompetitive collection of supplied vapor species between
adjacent nanowires. The similar phenomenon was seen in
MgO nanowire growth as reported by Annop et al.53 As such,the variation of the competitive collection and the VS
nucleation rate might coexist at the temperature condition
over 700 °C. Note that the decrease of nanowire density withdecreasing the growth temperature might be related to thesolidiﬁcation of Au metal catalyst. Since the melting point ofbulk Au is 1064 °C and the eutectic point of Au−Ni is 955 °C(for 82:18 of Au:Ni ratio), the temperature condition of 600 °C
might be low enough to solidify the Au catalyst. Once the metal
catalyst solidiﬁes, the growth mechanism of oxide nanowires
relies on the VSS growth.52 This might be another reason why
the growth rate decreases below 700 °C. Thus, there might be
the competition of VSS and VLS growth mechanisms in thecurrent NiO nanowire growth. The VSS contribution tends tobe dominant as the growth temperature decreases. On theother hand, as the growth temperature increases, the VLS
contribution might be enhanced. Since the phase states ofnanoscale metal catalysts somehow diﬀer from those of bulk
materials, it is necessary to experimentally observe the phasestate of nanoscale metal catalysts when varying the growthtemperature, for example, by using an in situ TEM method oran in situ X-ray method. Although this is a fascinating issue for
future work, this is beyond our current scope of this work. To
study the crystallinity of low temperature grown NiO
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based on the suppression of the vapor−solid (VS) nucleation at
the substrate surface, which promotes the crystal growth ofnanowires only at the liquid−solid (LS) interface. Manipulating
the supplied material ﬂuxes (oxygen and metal) and the growthtemperature enables us to enhance the nucleation only at the
LS interface. Furthermore, this strategy allows us to reduce thegrowth temperature of single crystalline NiO nanowires via the
VLS route down to 550 °C, which is the lowest growthtemperature so far reported. In addition, the fabricated single
crystalline NiO nanowires exhibited the superior memristive
properties.

■

AUTHOR INFORMATION

Corresponding Author*E-mail: yanagida@cm.kyushu-u.ac.jp. Phone: +81-92-5838829. Fax: +81-92-583-8820.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This study was supported by CREST and Grant-in-Aid forYoung Scientists (A) (No. 26706005) and ImPACT. K.N. was
supported by Grant-in-Aid for Challenging Exploratory
Research (No. 24651138). This work was performed under
the Cooperative Research Program of “Network Joint ResearchCenter for Materials and Devices”.

Figure 6. (a) FESEM image of NiO nanowire device. A single NiO
nanowire was bridged between Pt electrodes. The gap size was 200
nm. (b) I−V curves of the NiO nanowire device measured at room
temperature in air condition. The compliance current (cc.) of 10−9 A
was applied to prevent the destructive electric breakdown. (c) ON
state resistances as a function of current density for various sized NiOnanowires (30 nm, 80 nm, 100 nm). Readout voltage is 2 V. (d) The
distribution of ON and OFF state resistances for various sized NiOnanowires. The data is taken from 100 I−V curves. For thecomparison, the data from the polycrystal formed NiO nanobeam(diameter of 100 nm) is also shown. The standard deviation for both
RON/AveRON and ROFF/AveROFF is noted, which clearly shows thesmall distribution of ON and OFF resistance states in single crystalline
NiO nanowire devices.

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The resistances were measured at 2 V. As seen in Figure 6c, theelectrical conductions of NiO nanowire devices are uniformly
distributed into nanowires. In addition, the switching stabilityof the present single crystalline NiO nanowires is superior toother NiO nanostructure memristors, as shown in Figure 6d.
Figure 6d shows the statistical distributions of ON state
resistance and OFF state resistance, which are critical issues inindustrial applications of nonvolatile memory devices. For
comparison, data of polycrystalline NiO nanomemristors areshown. As can be seen, the statistical variation of single
crystalline NiO nanowires, especially the small diameter (30nm) NiO nanowires, is much smaller than the others. Although
the exact mechanisms in the current NiO nanowire memristors
should be further examined, the present experimental results
highlight the importance of single crystallinity on the nanoscale
memristive switching. The absence of grain boundaries seems
to be responsible for the observed memristive properties ofsingle crystalline NiO nanowires superior to polycrystalline
nanowires. This eﬀect of single crystalline nanostructures
suppresses the variation of ON or OFF state resistances via
limiting the conducting path in nanoscale.38,39 Thus, thefabricated single crystalline NiO nanowires oﬀer an opportunity
to develop novel functional nanodevices.

4. CONCLUSION
In conclusion, we demonstrated a rational strategy to fabricate
single crystalline NiO nanowires via a vapor−liquid−solid(VLS) route, which essentially allows us to tailor the diameter,
the spatial position, and the heterostructures. Our strategy is
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