JOURNAL OF APPLIED PHYSICS 111, 104305 (2012)

Growth of metal and metal oxide nanowires driven by the stress-induced
migration
Mingji Chen, Yumei Yue, and Yang Jua)
Department of Mechanical Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan(Received 13 February 2012; accepted 16 April 2012; published online 18 May 2012)
High quality Al and CuO nanowries are fabricated by simply heating the Al and Cu samples in air.
Although the experimental operations and the stress-induced migration processes are quite similar,
the causes of the driving forces and the growth mechanism are completely different. For the growthof Al nanowires, the driving force is determined to be the compressive stresses caused by thethermal expansion mismatch between Al film and Si substrate, and the growth mechanism is
proposed to be the extrusion of atoms from the bases of nanowires (EAFB). For the growth of CuO
nanowires, the driving force is determined to be the compressive stresses caused by the formationof Cu oxide layers, and the growth mechanism is proposed to be the formation of oxide moleculeson surfaces of the nanowires (FOOS). The direct experimental observations of both EAFB and
FOOS are presented. It is also demonstrated that stress distribution on the macroscopic level,
which is caused by thermal or mechanical manipulation, can also influence the growth of CuO
nanowires, which makes it prospective to control the growth of metal oxide nanowires by
C 2012
designing the stress distribution within the sample from which the nanowires are generated. V
American Institute of Physics. [http://dx.doi.org/10.1063/1.4718436]
I. INTRODUCTION

Metal oxide nanowires, such as CuO and ZnO nanowires, were demonstrated to have fantastic applications insolar cells,1 field emitters,2 toxic gas detectors,3 etc. Compared with traditional semiconductor such as Si and GaAs,
metal oxides are more plentiful and low-cost with low toxicity and good environmental acceptability, which makes devices based on metal oxide nanowires more prospective.
Synthesis of dense CuO nanowires by heating the highpurity Cu samples was reported in 2002.4 This method was
followed by many researchers5–12 due to the single crystal or
bicrystal structures of the resultant nanowires as well as its
simple requirements on equipments and experimental conditions. Moreover, it can serve as an effective method for positioned growth of well aligned metal oxide nanowires11 andthus is potential for integrating nanowires into devices.2
In fact, growth of metal oxide nanowires or nanowhiskers during the heating process of metals has been
observed since 1950 s.13–15 Cu, Zn, and Fe have beenreported to be three of the most typical metals from which
CuO,4,14 ZnO,14,16 and Fe2O3 (Refs. 14 and 17) nanowires
can be steadily generated, respectively. The word “steadily”
here indicates that the metal oxide nanowires can be generated during heating of the high-purity metal samples, regardless of the types and dimensions of samples. On the other
hand, Al,18–20 Ag,21 and Bi (Refs. 22–24) nanowires have
only been reported to be generated by heating their filmsubstrate systems.
Beside plenty of synthesis and application researches,
the growth mechanism has also been studied. Some researcha)

Author to whom correspondence should be addressed. Electronic mail:ju@mech.nagoya-u.ac.jp. Tel.: þ81-52-789-4672. Fax: þ81-52-789-3109.

0021-8979/2012/111(10)/104305/6/$30.00

ers tended to believe that the nanowires were formed due tothe extrusion of metal atoms driven by the compressive
stresses in the samples.18–25 In this case, the nanowires were
supposed to grow from their bases. On the other hand,
growth of metal oxide nanowires was also interpreted as theresult of thermal oxidation process.5–10,16,17,26 In this case,the nanowires were supposed to grow by adding oxide molecules on their tips, which was quite different from the extrusion mechanism. Therefore, up to now, there is still nogenerally accepted interpretation on fabricating metal andmetal oxide nanowires by simply heating the high-purity
metal samples.
In this paper, we present reasonable interpretations on
the growth mechanism of metal and metal oxide nanowires.
The growth of nanowires is driven by stress-induced migration, but the growth mechanism can be classified into two
different types, namely, the extrusion of atoms from thebases of nanowires (EAFB) and formation of oxide molecules on surfaces of the nanowires (FOOS). Although growthof metal oxide nanowires from their tips was supported bymany researchers,5,6,8–10,16,17,26 the experimental observationof FOOS was rarely reported. Here, we present direct experimental observation on the growth process of both metal andmetal oxide nanowires to support our interpretations on the
different types of growth mechanism. Although it is believedthat gas flow can affect the growth of nanowires,5 in this paper, we only focus on the growth of nanowires in air. It isshown that the metal oxide nanowires can grow very welleven without a closed environment and specific gas flow.
II. EXPERIMENT

Growth of Al and CuO nanowires was carried out in thisstudy on both the metal film-substrate systems and metal

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J. Appl. Phys. 111, 104305 (2012)

The samples with nanowires were observed by scanningelectron microscopy (SEM). Individual nanowires were also
separated from the substrates and collected on the observation grid of the transmission electron microscope (TEM).
The results of electron energy loss spectroscopy (EELS) together with the TEM images were obtained from the individual nanowires.
III. RESULTS AND DISCUSSION
A. Growth of Al nanowires dominated by the EAFB
mechanism

FIG. 1. Al nanowires growing after heating at 240  C for 4.5 h. (a) A nanowire with high aspect ratio. (b) A typical Al nanowire with diameter of
500 nm. (c) A thick and relatively short nanowire. (d) Number density of the
Al nanowires, the scale bars in all the insets are 5 lm.

foils. The Al and Cu foils utilized were 250 and 300 lm
thick, respectively, and the purity was 99.9%. Al film withthickness of 200 nm was deposited directly on the Si substrate by the electron beam evaporation (EBE). For Cu film,
a 60 nm thick Ta layer was first deposited on the Si substrate
to serve as the adhesive layer. Then, a 400 nm thick Cu film
was deposited on the Ta adhesive layer by EBE. Patterned
Cu film samples were also prepared by implementing the optical lithography before deposition of the Ta layer and a
400 nm thick Cu film. The patterned Cu film was in the shapeof a flat column with diameter and thickness of 10 lm and
400 nm, respectively.
In order to grow nanowires, the metal foils and films
were heated on a ceramic heater in air. The ceramic heater
was powered by a voltage source, and the heating temperature could be well controlled by adjusting the supplied voltage. The Al foils and films were heated at temperatures from
200 to 300  C, while the Cu foils and films were heated attemperatures from 250 to 450  C.

Al nanowires can grow well only on the Al film samples
at a narrow temperature range between 200 and 240  C.
Heating Al foils did not generate nanowires, while heating
Al films at a temperature out of the aforementioned range
only generated nanowires occasionally on the film surfaces.
Heating Al films at 240  C for 4.5 h gave the following
results. The number density of Al nanowires was approximately 103 cm2. The length and diameter of the Al nanowires were always 10–50 lm and 200–500 nm, respectively,
as shown in Fig. 1. The highest aspect ratio was around
200–300. Shorter nanowires and nanowires with diametersof smaller than 200 nm or larger than 500 nm were also
found occasionally.
The growth process of Al nanowires was observed andpresented in Figs. 2(a)–2(d). As shown in Figs. 2(a) and 2(b),the tip of nanowire did not change during the growth process.
Thus, it was demonstrated by direct experimental observationthat the Al nanowires grew from their bases. The straight andfacet nanowire surfaces shown in Figs. 1 and 2 correspondedwith the shapes of the holes surrounding the bases of nanowires. It can also be seen from Fig. 2(c) that the morphologyof the nanowire tip corresponded well with that of the local
protective layer, which was peeled off from the sample surface (Fig. 2(d)). All these evidences support the conclusionthat the growth mechanism of Al nanowires belongs to EAFB.
When the Al film-substrate system was heated in air, inplane biaxial compressive stresses were generated in Al film

FIG. 2. Growth process of the Al nanowire. (a) After heating for 1.5 h. (b) After heating for 3 h. (c) The tip of nanowire. (d) The base of nanowire. (e) Illustration of the EAFB mechanism for Al nanowire growth.

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due to the thermal expansion mismatch between the film andthe substrate. Since the surface of Al film was covered by athin protective layer, either a deposited protective layer18 or
the natural oxide layer (as in this study), the compressive
stresses could not be relieved freely on the film surface. The
Al atoms tended to migrate from more compressive areas toless compressive ones in the form of stress-induced grainboundary diffusion and surface diffusion (Fig. 2(e)). When
plenty of Al atoms accumulated on some sites near the film
surface, it was resulted that the local protective layers werebroken through and nanowires were extruded from these broken sites. Thus, the shapes of the nanowire surfaces should
follow the shapes of the holes from which they were
extruded. This is the reason why the facet nanowire surfaces
corresponded with the shapes of the holes surrounding thebases of nanowires. Then, the continuous growth of Al nanowires was supported by the continuously coming atoms due
to stress-induced migration. In the whole process, the stressgradient induced by the thermal expansion mismatch
between Al film and Si substrate acted as a dominant factor.
That is the reason why Al nanowires could not been generated by heating Al foil in air.
B. Growth of CuO nanowires dominated by the FOOS
mechanism

SEM images of CuO nanowires growing on Cu films
and Cu foils, which were heated at the temperature from 300
to 450  C for 2 h were shown in Figs. 3 and 4, respectively. It
can be seen from these two figures that CuO nanowires weregenerated on both film and foil samples when heating thesamples in air at a proper temperature range. CuO nanowires
growing on Cu foils after being heated at 400  C for 2 h wereeven longer than those growing on Cu films under the same
condition. This is quite different from the case of Al nanowires. The number density of CuO nanowires may reach up
to 106 times larger than that of the Al nanowires. Besides,
for CuO nanowires growing on both film and foil samples, it
was observed that the diameter of nanowires increased withthe temperature obviously. From Figs. 3 and 4, the optimum

J. Appl. Phys. 111, 104305 (2012)

FIG. 4. CuO nanowires growing on the Cu foils after being heated at different temperatures of (a) 300  C, (b) 350  C, (c) 400  C, and (d) 450  C for 2 h.

temperature for CuO nanowire growth was supposed to bearound 400  C, while much thinner nanowires could be generated at 350  C. For heating temperatures of 300  C or
lower, only thin and short (0.5 lm long) nanowires could
be generated. For heating temperatures of 450  C or higher,
the number density became very low. The data of CuO nanowires growing on Cu films were presented in Table I.
To study the influence of heating time, Cu films were
heated at 400  C for 10, 30, 60, and 120 min, respectively,
and the results were presented in Fig. 5. Although the number density was almost unchanged, the length and diameterof CuO nanowires both increased with the heating time. It
can be seen from Fig. 6 that the increasing rates of lengthand diameter were relatively high before the film samples
were heated for 30–60 min, and then became much lower.
Heating the film samples for more than 2 h no longer
improved the growth of CuO nanowires significantly.
Direct observation of growth process of CuO nanowires
on the same film sample was also carried out. As shown in
the enlarged SEM images in Figs. 7(a) and 7(c), most nanowires were observed to grow directly from the surface grainsof the sample. This is quite different from the Al nanowires,
which were extruded from holes on the sample surface.
Moreover, the shapes and surface morphologies of the CuO
nanowires were much more complicated than the Al nanowires. Most of the CuO nanowires were in the shape of aneedle, namely, thicker at their bases and thinner at theirtips. With the advance of the growth process, branched nanowires (VI in Fig. 7(b)) and nanowires having secondary parts(more narrow parts on the thicker bases, as I and III in
Fig. 7(b)) were also observed. Images shown in Figs. 7(b)TABLE I. Data of CuO nanowires growing on Cu films after being heatedat different temperatures for 2 h.
Temperature/ C
9FIG. 3. CuO nanowires growing on the Cu films after being heated at different temperatures of (a) 300  C, (b) 350  C, (c) 400  C, and (d) 450  C for 2 h.

2Number density/10 cm
Average length/lm
Average diameter/nm

300

350

400

450

1.5
0.5
15

1.8
1.6
30

1.5
3.5
48

0.4
2.0
100

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FIG. 6. Curve of (a) length and (b) diameter of CuO nanowires vs the heating time. The Cu nanowires were generated on Cu films at 400  C.

FIG. 5. CuO nanowires growing on the Cu films after being heated at
400  C for (a) 10 min, (b) 30 min, (c) 60 min, and (d) 120 min.

and 7(d) were taken from the same places as Figs. 7(a) and
7(c), respectively, after the sample had been heated for 20
more minutes. Obvious elongation of the secondary parts (Iand III in Fig. 7(b)) and the formation of branched structure(VI in Fig. 7(b)) indicated that the CuO nanowires did not
grow from their bases. It has also been observed that many
nanowires grew not only longer but also thicker (II, IV in
Figs. 7(a) and 7(b) and I, II, III in Figs. 7(c) and 7(d)). These
evidences support that the growth of CuO nanowires was
implemented by the formation of oxide molecules on surfaces (tips and sidewalls) of the nanowires (FOOS). Although
growth of CuO nanowires from their tips were supported bymany researchers,5,6,8–10,26 the direct experimental observation of FOOS as presented here was rarely reported before.
The driving force for growth of CuO nanowires was proposed to be the compressive stresses generated in the samples
during the heating process.8,10,25,26 However, regarding to theorigin of the compressive stresses, there was still no generally
accepted interpretation. Some researchers indicated that thestresses were generated due to the formation of Cu2O andCuO layers during the heating process,8,10,26 while others
thought that it was the thermal expansion mismatch between
Cu film and Si substrate that induced the compressive stresses
in the Cu film.25 According to our experiment results thatheating Cu foils also generated CuO nanowires, we believedthat the former interpretation was more reasonable.
When the Cu samples were heated in air, two thermodynamically stable oxide layers, namely, Cu2O and the topmost
CuO layers, formed.26 Since the molar volumes were in theorder of Cu < Cu2O < CuO, stresses were generated in thesamples. The Cu2O layer suffered compression from Cu on
its bottom surface and tension from CuO on its top surface,
while the CuO layer suffered compression from Cu2O on its
bottom surface and had a stress-free top surface. Thus, thestress gradient was in the direction from the Cu/Cu2O interface to the CuO surface (outward of the sample surface). The
atomic flux that is caused by stress-induced migration can be
described as25
J¼

CXD
rr;
kB T

(1)

where C is the atomic concentration, X is the atomic volume,
kB is Boltzmann’s constant, T is the absolute temperature, D
is the local diffusion coefficient, and r is the hydrostatic
stress (average normal stress). It can be seen from Eq. (1)that the gradient of hydrostatic stress can serve as the driving
force for atomic diffusion and the atomic flux is in the direction from more compressive/less tensile area to less compressive/more tensile area. Therefore, with the formation of
Cu2O and CuO layers, Cu ions migrated from the Cu/Cu2O
interface to the sample surface due to the stress-induced
grain boundary diffusion, as shown in Fig. 7(e). Thesemigrating ions served as the continuous source for the formation of CuO nanowires.

FIG. 7. Growth process of CuO nanowires. (a)and (c) SEM images of nanowires taken after
heating the sample for 10 min. (b) and (d) SEM
images taken from the same places as (a) and(c), respectively, after heating the sample for
30 min. The Cu film sample was heated at
350  C. (e) Illustration of the FOOS mechanism
for CuO nanowire growth.

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When the Cu ions arrived at the sample surface, they
continued to migrate to some specific sites through surface
diffusion and initiated the growth of CuO nanowires. It wasnoted from Figs. 7(a) and 7(c) that most CuO nanowires initiated on the surfaces of CuO grains rather than out of thegrain boundaries, which were in consistent with the statement made in Ref. 26. The driving forces for the surface diffusion were assumed to be the concentration gradient of the
Cu ions and the local stress gradient on a microscopic level
due to the material singularities.25 After the initiation of
nanowires, the subsequently arriving Cu ions climbed along
the sidewalls of the nanowires, as illustrated in Fig. 7(e). The
uneven nanowire surface (V in Fig. 7(b)) was an evidence of
this climbing process. During the climbing process, the Cu
ions might be incorporated into the CuO nanowires by reacting with the oxygen. This is the reason why the nanowires
grew not only longer but also thicker with the increase of theheating time. It should be mentioned that, heating Cu foil at
400  C for more than 2 h generated much longer nanowires
than heating Cu thin film under the same condition (comparison between Figs. 3(c) and 4(c)). The reason is that the 400 nm
thick Cu film was totally oxidized after being heated for a longtime, and thereby the stress gradient which was the driving
force for Cu ion migration tended to disappear. Another reason
is the lack of Cu ion source in the Cu thin film.
C. Examination on components of the nanowires

To examine the components of the nanowires, individual
nanowires were separated from the substrates and collectedon the TEM observation grid. The TEM images and EELS
spectra were obtained from the individual nanowires andshown in Fig. 8.
As shown in Fig. 8(a), the surface of the nanowire collected from the Al film sample was straight and smooth. The
lattice fringe spacing shown in Fig. 8(b) was measured to beapproximately 0.230 nm, which corresponded to the {111}plane of the Al crystal. It was also noticed from Fig. 8(b)that there was a 2 nm thick layer (without clear fringe) surrounding the Al nanowire. This thin layer was deduced to be

J. Appl. Phys. 111, 104305 (2012)

Al2O3 since pure Al is easy to be oxidized in air and forms athin but dense surface layer to prevent further oxidation. The
EELS spectrum shown in Fig. 8(c) confirmed that the main
component of this nanowire was the Al metal. A CuO nanowire with diameter of 50 nm was shown in Fig. 8(d). The lattice fringe spacing shown in Fig. 8(e) was measured to beapproximately 0.253 nm, which corresponded to the {002}plane of the CuO crystal. The EELS spectrum shown in Fig.
8(f) corresponded well with the typical CuO spectrum, whichfurther confirmed that the main component of this nanowire
was CuO.
The component examination supported that the nanowires growing from the Al film were Al nanowires, while
those growing from the Cu film and Cu foil were CuO nanowires. This conclusion was consistent with our interpretations on the EAFB and FOOS growth mechanism.
D. Nanowires growing on patterned Cu film

As mentioned in Sec. III B, it was the stresses generated
by the formation of Cu2O and CuO that dominated thegrowth of CuO nanowires. However, in the case of Cu film
sample, compressive stresses were also induced in the Cu
film due to the thermal expansion mismatch between Cu film
and Si substrate. In this section, the influence of such stresseson the growth of CuO nanowires was investigated.
In our experiment, it was observed that CuO nanowires
preferred to grow on the edge of the patterned Cu film, as
shown in Fig. 9(a). To interpret this phenomenon, finite element (FE) simulation was carried out to obtain the distribution of compressive stresses in Cu film, which were inducedby the thermal expansion mismatch between Cu film and Si
substrate.
To simplify the calculation without loss of correctness,the patterned Cu film-substrate system was modeled as follows. A Ta disc of 10 lm in diameter and 60 nm in height
was placed on a Si substrate. Then a Cu column (patterned
Cu film) of 10 lm in diameter and 400 nm in height wasplaced on the Ta disc. Young’s moduli of 110, 185, and
150 GPa, Poisson’s ratios of 0.34, 0.34, and 0.28, and

FIG. 8. TEM and EELS results of the individual nanowires. (a) TEM image of an Al nanowire. (b) High resolution TEM image from apart of the nanowire shown in (a). (c) EELS
spectrum of the Al nanowire. (d) TEM image ofa CuO nanowire. (e) High resolution TEM
image from a part of the nanowire shown in (d).
(f) EELS spectrum of the CuO nanowire.

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FIG. 9. The relationship between growth of CuO nanowires and the stresses
induced by thermal expansion mismatch. (a) Nanowires growing on patterned Cu film. (b) Distribution of the compressive stresses obtained by the
FE simulation.

thermal expansion coefficients of 16.5  106 K1, 6.3
 106 K1 and 2.6  106 K1 were adopted for Cu, Ta,
and Si, respectively. Tie constraint was adopted as theboundary condition between every two adjacent layers. It
can be estimated that Cu and Ta may experience the plastic
deformation due to the great difference between the thermal
expansion coefficients of metal and the Si substrate. Therefore, the ideal elastic-plastic constitutive relation was
adopted for Cu and Ta in this study, and the yield stress was
set to be 170 MPa. The initial temperature of the model
was set to be 298 K. Then, the temperature was elevated to623 K. The temperature was assumed to be uniformly
applied to the model according to the experimental condition. In fact, only the stress status at the elevated temperatureof 623 K was calculated.
As can be seen from Fig. 9(b), the radial distribution ofhydrostatic stress on the top surface of the Cu column is insuch a way that it is less pressed on the edge than in the center. The stress gradient is in the direction from the center of
the Cu column to its edge. Thus, according to Eq. (1), Cu
ions tend to migrate along the stress gradient to the lesspressed edge of the patterned Cu film. This is the reason why
CuO nanowires preferred to grow on the edge of the patterned Cu film. In a very recent paper, it was also reported
that CuO nanowires preferred to grow on the tensile surface
of a bended Cu foil.27 Thus, it can be further deduced that
although the growth of metal oxide nanowires is dominatedby stresses that arise due to the formation of metal oxide
layers, stress distribution on the macroscopic level, which is
caused by thermal or mechanical manipulation also influences the growth situation. It is prospective to control thegrowth of metal oxide nanowires by designing the stress distribution within the sample from which the nanowires are
generated.
IV. CONCLUSION

In conclusion, we studied the growth mechanism ofmetal and metal oxide nanowires through implementing the
growth of Al and CuO nanowires by simply heating the Al
and Cu samples in air. Although the experimental operations
and the stress-induced migration processes were quite similar,
the causes of the driving forces and the growth mechanism

J. Appl. Phys. 111, 104305 (2012)were completely different. For the growth of metal (Al) nanowires, the driving force was determined to be the compressive
stresses caused by the thermal expansion mismatch between
metal film and Si substrate, and the growth mechanism was
proposed to be the extrusion of atoms from the bases of nanowires (EAFB). For the growth of metal oxide (CuO) nanowires, the driving force was determined to be thecompressive stresses caused by the formation of metal oxide
layers, and the growth mechanism was proposed to be the formation of oxide molecules on surfaces of the nanowires(FOOS). The direct experimental observations of both EAFB
and FOOS were presented. It was also demonstrated that
although the growth of metal oxide nanowires was dominatedby stresses that arose due to the formation of metal oxide
layers, stress distribution on the macroscopic level, which
was caused by thermal or mechanical manipulation also influenced the growth situation. It is prospective to control thegrowth of metal oxide nanowires by designing the stress distribution within the sample from which the nanowires are
generated.
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