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Scripta Materialia 66 (2012) 81–84
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Stress-induced growth of well-aligned Cu2O nanowire arrays andtheir photovoltaic eﬀect
Yumei Yue, Mingji Chen, Yang Ju⇑ and Lan Zhang
Department of Mechanical Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Received 15 July 2011; revised 29 September 2011; accepted 29 September 2011
Available online 6 October 2011

A well-aligned Cu2O nanowire array was fabricated by the stress-induced method. A new stress-redistribution phenomenon
related to the cooling procedure was observed and a creative growth procedure was demonstrated. High-quality Cu2O nanowires
with an aspect ratio up to 300 and a growth density higher than 109 cm2 can be derived under the optimum condition of cooling
gradually for 4 h after heating for 5 h. A typical photovoltaic eﬀect was demonstrated to exist in the Cu2O/Cu junction.
Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Nanostructure; Whiskers; Thin ﬁlms; Diﬀusion; Stress-induced growth

Cuprous oxide (Cu2O) and cupric oxide (CuO)
nanowires are known to have many interesting physicalproperties and extensive applications [1,2]. Comparedwith traditional semiconductor materials, such as Si
and GaAs, copper oxide is more plentiful and low-costwith low toxicity and good environmental acceptability.
Thus, the synthesis and applications of Cu2O and CuOnanowires are of particular interest for researchers.
The most common method to fabricate Cu2O andCuO nanowires is the template-based synthesis in solution [3,4]. Although these template-based methods areeﬀective in preparing nanowires with controllable diameters, they usually lead to complicated processes and theyield depends heavily on the density of channels. Reduction and dehydration of Cu(OH)2 nuclei formed in solutions can also give birth to Cu2O [1,5,6] and CuO [7]nanowires, respectively. However, the as-obtained copper oxide nanowires are always polycrystalline ones chaotically distributed in the solution.
A thermal annealing method for synthesis of CuO
nanowires was proposed by Jiang et al. in 2002 [8]. Thismethod takes advantage of a simple procedure and highyield performance, and the as-obtained CuO nanowires
are of orderly distribution, high quality and ﬁne crystal
structures such as bicrystal [8] and single crystal [9,10].
As a result, it has become one of the most popular methods for fabricating CuO nanowires [11–14]. However, to⇑ Corresponding author. E-mail: ju@mech.nagoya-u.ac.jp

the best of our knowledge, synthesis of Cu2O nanowiresby the high-yield thermal annealing method has not yet
been reported.
Recently, the growth of nanowires from electron beam(EB) evaporated Cu thin ﬁlms on the SiO2/Si substrate at
340 °C or even lower temperatures was reported, where
the products were described as Cu nanowhiskers though
no detailed investigation of their morphology and microstructures was performed [15]. Keeping the relatively low
growth temperature in mind, we demonstrate in this paper the on-chip growth of Cu2O nanowires by the thermal
stress-induced method, with the emphasis on the eﬀect ofcooling procedure. Nanowires obtained under the optimum heating and cooling conditions can reach up to
10 lm long, about six times longer than those reported
in Ref. [15].
The samples from which the Cu2O nanowires grew
were fabricated as follows. A Ta layer of 60 nm in thickness was deposited on a 280 lm thick Si [1 0 0] substrate
covered with a thin SiO2 layer by the EB evaporation
technique. This Ta layer can enhance the adhesion between the SiO2 layer and the subsequently deposited
Cu ﬁlm and prevent the diﬀusion of Cu atoms into otherlayers. A Cu ﬁlm was then deposited on the Ta layer by
EB evaporation to form the Cu/Ta/SiO2/Si multilayer
sample. The typical thickness of the Cu ﬁlm is 400 nm.
In order to produce thermal stress in the Cu ﬁlm which
is needed for stress-induced growth of nanowires, thesample was heated on a ceramic heater in air. The room
temperature was kept at 25 °C. By controlling the voltage

1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.scriptamat.2011.09.041

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Y. Yue et al. / Scripta Materialia 66 (2012) 81–84

of the power supply, the heating temperature can be
adjusted to the required value. A large heating rate of63 °C min1 was adopted in our experiment, by which
the constant temperature of 340 °C was achieved 5 minafter turning on the power of the heater. After being
heated for 4–9 h at 340 °C, the samples were cooled in
air either directly or gradually for 3, 4 or 5 h, respectively,
to investigate the eﬀect of cooling procedure on nanowire
growth. In the gradual cooling process, the voltage of thepower supply was decreased step by step to produce thegradually reduced temperature during a certain period.
The temperature proﬁle during the 5 h heating and cooling (either directly or gradually for 4 h) process is shownin Figure 1a. The cooling rate during the 4 h cooling process is 1.31 °C min1.
The macroscopic stress over the whole Cu ﬁlm during
the heating and gradual cooling process was measured
by means of a wafer curvature system using interference
between two laser paths split from the same laser beam(k = 632.8 nm). The stress can be derived from the measured curvature using Stoney’s formula.
The samples with nanowire arrays were observed byﬁeld emission scanning electron microscopy (FE-SEM),
energy dispersive X-ray spectroscopy (EDXS) and
X-ray diﬀraction (XRD). Then, the fabricated nanowires were dispersed onto a Cu grid and observed by
the transmission electron microscopy (TEM), selectedarea electron diﬀraction (SAED) and high-resolution
transmission electron microscopy (HRTEM) to examine their microstructure and composition.

Figure 1. Growth of nanowires: (a) temperature proﬁle during the 5 hheating and cooling (either directly or gradually for 4 h) process, (b)nanowires growing on the substrate in the experiment with direct
cooling process, (c) side view of (b), (d)–(f) nanowires growing on
hillocks during the gradual cooling process lasting for (d) 3 h, (e) 4 h
and (f) 5 h, respectively.

The samples for the photovoltaic eﬀect investigation
were prepared as follows. First, a wire was connectedto the top surface of the Cu/Ta/SiO2/Si sample by thehigh-temperature conductive adhesive (Pyro-Duct
597A, Oludec Ltd.), immediately after the EB evaporation process. Then the sample endured a heating andgradual cooling treatment as shown in Figure 1a, andthe on-chip Cu2O/Cu multilayer structure with hillocks
and Cu2O nanowires formed. Another wire was thenconnected to the surface of the Cu2O layer by the conductive adhesive.
After being heated at 340 °C for 4–9 h and cooled directly, the sample was observed to be covered with dense
nanowires, as shown in Figure 1b. Figure 1c shows theside view of nanowires growing on the substrate. The
average length of these nanowires is 2 lm and their
growth density is 109 cm2.
For the samples that underwent the gradual coolingprocess, some randomly distributed hillocks with much
longer nanowires on them were observed, as shown in
Figure 1d–f. This phenomenon did not appear for samples under direct cooling, even though these samples
were heated for 9 h and cooled directly. FE-SEM analysis showed that the nanowires growing on the hillocks
were 10–50 nm in diameter. For samples treated by the
optimum heating and cooling procedure (Fig. 1e), nanowires growing on the hillocks can reach up to 10 lm
long, nearly ﬁve times longer than those growing on
the substrate. The largest aspect ratio was not less than
300 and the growth density was even higher than 109
cm–2. This phenomenon has not been reported elsewhere,
and it provides a positive view for the fabrication of onchip copper oxide nanowires with much higher aspectratio.
The diﬀerent growth mechanism of the nanowires on
the substrate and the hillocks is discussed as follows.
When the as-fabricated multilayer sample was placedin air at room temperature, the surface of the Cu ﬁlm
was oxidized, forming a thin layer of Cu2O. During
the heating process, all layers experienced thermal
expansion. Since the thermal expansion coeﬃcients ofdiﬀerent layers are quite diﬀerent (2.6  106 K1 for
Si and 16.5  106 K1 for Cu at 25 °C), circumferential
tensile or compressive stresses occurred in all the layers
to keep the deformation compatibility of the whole sample. Therefore, macroscopic compressive stresses weregenerated in the Cu layer, as shown in Figure 2a andd. In the early stage of the heating process, the surface
diﬀusion of Cu atoms near the Cu2O/Cu interface wascaused by the stress concentrations which occurred at
the grain boundaries due to material singularities. Whenthe diﬀusion rate was high enough and the accumulationof Cu atoms at the Cu2O/Cu interface attained a criticalvalue, the accumulated Cu atoms started to penetrate
the Cu2O layer via any weak spots, after which the
nanowires were nucleated, as shown in Figure 2b. The
growth of nanowires relieved parts of compressive stresses near the Cu2O/Cu interface and the stress gradient
between the top (Cu2O side) and bottom (Si side) facesof the Cu ﬁlm was generated.
During the cooling process, the stresses induced in
the sample due to the thermal expansion tended to disappear and the sample tended to recover to the state

Y. Yue et al. / Scripta Materialia 66 (2012) 81–84

83

Figure 2. Schematics and supporting data for the growth of nanowires: (a) the compressive stresses generated in the Cu ﬁlm by heating, (b) growth ofnanowires on the substrate, (c) growth of hillocks and nanowires on them, (d) the macroscopic stresses over the whole Cu ﬁlm measured by laserinterference during the heating and gradual cooling process.

before it was heated, as shown in Figure 2d. However,since the growth of nanowires had relieved a lot of thecompression stresses in the Cu ﬁlm from the Cu2O/Cu
interface, a local tensile stress was generated in the Cu
ﬁlm near the Cu2O/Cu interface in the contraction process of the Cu ﬁlm. Consequently, a larger stress gradient was formed between the top and bottom faces of theCu ﬁlm, which might further increase the diﬀusion rateof Cu atoms moving from the bottom to top faces of
the Cu ﬁlm.
During the gradual cooling process, which lasted for along time, a huge diﬀusion of Cu atoms was realized andthe hillocks were formed, as shown in Figure 2c. Whenthe growth rate of hillocks was too high, which corresponded to a relatively high cooling rate, the formationof surface oxide layers cannot catch up with the growthof hillocks, and thereby little nanowires grew on the hillocks, as shown in Figure 1d. When the cooling rate was
too low, the stress gradient might not be large enough forthe rapid diﬀusion of Cu atoms and the growth of hillocks cannot match with the formation of thick surface
oxide layer, thereby, fewer nanowires were obtained(Fig. 1f). Only when the gradual cooling was held in aproper rate, can dense and long nanowires grow on thehillocks, as shown in Figure 1e. This stress-redistribution
phenomenon, which can promote the growth of Cu2O
nanowires, has not been reported elsewhere. After many
experiments, the optimum growth condition of heatingfor 5 h and cooling gradually for 4 h was proposed. On
the other hand, when the sample was cooled directly,
more cracks instead of hillocks were formed on the sample surface due to the extremely high cooling rate.
Figure 3a shows the TEM image of an individual
nanowire with the smooth surface and the uniform
diameter along the axis. From the SAED pattern takenfrom a random assembly of the nanowires, as shown in
Figure 3b, the compositions can be deduced to be Cu2O
and CuO, of which the former is the main composition.

Figure 3c shows the HRTEM image of a single-crystal
area with clear lattice fringes. The lattice fringe spacingis 0.30 nm, which corresponds to the {1 1 0} plane of
Cu2O crystal. As shown in Figure 3d, the EDXS spectrum from the surface of a hillock indicates that themain chemical elements are Cu and O, the percentagesof which are 67.3% and 32.4% (nearly 2:1), respectively.
This result is in accordance with the SAED pattern thatthe composition of the as-obtained nanowire is mainly
Cu2O. XRD analysis on the surface of a sample that
had undergone gradual cooling also shows that the main
composition of the sample surface and nanowires covering on it is Cu2O (Fig. 3e). It should be noted that since
Cu2O is a metastable phase, some of the nanowires
growing on the substrate which experienced the annealing process at 340 °C may be further oxidized to be
CuO. This is the reason why some CuO was observedin Figure 3b and e. However, since the nanowires growing on hillocks are generated during the cooling process
at relatively low temperatures, they can hold the metastable phase of Cu2O.
Cu2O is a typical semiconductor, predicted to bepromising for photovoltaic applications [16]. The Schottky barrier forming on the Cu2O/Cu interface can playa similar role as the p–n junction to generate the photovoltaic eﬀect [17]. Thus, to further verify the composition of the as-obtained nanowires, the photovoltaic
eﬀect of the multilayer sample with nanowires wasinvestigated.
The rectifying behavior in the dark current curve indicates that a Schottky barrier is formed on the Cu2O/Cu
interface, as shown in the inset of Figure 4. To obtain thephotovoltaic current density–voltage (J–V) curve, thesample was illuminated by a lamp. The open circuit voltage (Voc) and the short circuit current density (Jsc) can beread from the photovoltaic J–V curve shown in Figure 4
to be 7.5 mV and 2.67 lA cm2, respectively. The ﬁllfactor FF ¼ ðJ  V Þmax =ðJ sc  V oc Þ  0:25 can also be

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Y. Yue et al. / Scripta Materialia 66 (2012) 81–84

Figure 3. Microstructures and compositions of the Cu2O nanowires: (a) TEM image of an individual nanowire, (b) SAED image taken from arandom assembly of the nanowires, (c) HRTEM image of part of an individual nanowire, (d) the EDXS pattern taken from the surface of a hillock,(e) XRD analysis on the surface of a sample that had undergone gradual cooling.

Cu2O nanowires with aspect ratios higher than 300 and
for further improving the growth density of nanowires.
An optimum growth condition of heating for 5 h andcooling gradually for 4 h is proposed. The as-obtained
Cu2O nanowires are mainly single-crystal ones withsmooth surfaces and uniform cross-sections along theiraxes. A typical photovoltaic eﬀect is demonstrated to exist in this Cu2O/Cu multilayer structure with Cu2O
nanowire array, which makes it a potential option forcheap and low-toxicity photovoltaic devices with goodenvironmental acceptability.
Figure 4. Measured photovoltaic properties of a multilayer sample
with Cu2O nanowires. The inset shows the structure of the sample andthe measurement circuit, as well as the dark current curve.

calculated from the J–V curve. As shown in Figure 4, theshape of the J–V curve is similar to that of the Cu2O/nZnO nanowire photovoltaic sample [18]. The typicalphotovoltaic eﬀect further demonstrates that the main
composition of the nanowires and the sample surface is
Cu2O rather than Cu [15] or CuO [13,14]. It should be
mentioned that the photovoltaic current density andvoltage of the present sample are relatively low. Thismay be due to the presence of some microcracks on thesample surface after the heating and cooling process,which brings up the inner resistance of the sample andleads to smaller photovoltaic current. It was also observed that, when the focused light scanned on the sample surface from one point to the others, the photovoltaic
voltages were diﬀerent. The area with more hillocks andwell-growing Cu2O nanowires always corresponded tohigher photovoltaic voltage. The direct cooling sample(without hillocks and well-growing nanowires) and thesample heated at a relatively low temperature (without
nanowires) were also studied, but no obvious photovoltaic eﬀect was observed. Therefore, the observed photovoltaic eﬀect of the sample is mainly contributed by thewell-growing Cu2O nanowires.
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