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Nano Res
1
Anelasticity of twinned CuO nanowires
Huaping Sheng, He Zheng, Fan Cao, Shujing Wu, Lei Li, Chun Liu, Dongshan Zhao, and Jianbo Wang ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0868-x
http://www.thenanoresearch.com on July 23, 2015
© Tsinghua University Press 2015
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Nano Research
DOI 10.1007/s12274-015-0868-x
1
TABLE OF CONTENTS (TOC)
Anelasticity of Twinned CuO Nanowires
Huaping Sheng§, He Zheng§, Fan Cao§, Shujing Wu, Lei
Li, Chun Liu, Dongshan Zhao, and Jianbo Wang*
School of Physics and Technology, Center for Electron
Microscopy and MOE Key Laboratory of Artificial Micro-
and Nano-structures, Wuhan University, Wuhan 430072,
China
§These authors contributed equally to this work.
An unexpected anelasticity was observed in CuO NWs with
twin structures during the mechanical loading-unloading cycles,
demonstrated by in situ TEM deformation experiments.
2
Anelasticity of Twinned CuO Nanowires
Huaping Sheng§, He Zheng§, Fan Cao§, Shujing Wu, Lei Li, Chun Liu, Dongshan Zhao, and Jianbo Wang*()
School of Physics and Technology, Center for Electron Microscopy and MOE Key Laboratory of Artificial Micro- and
Nano-structures, Wuhan University, Wuhan 430072, China
§These authors contributed equally to this work.
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)
© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT The mechanical behaviors of CuO nanowires (NWs) were investigated by in situ transmission electron
microscopy. During compression, the NWs exhibit high bending capability associated with high mechanical
stress. Interestingly, an unexpected anelastic behavior has been consistently observed after the stress releasing.
Further investigations indicate that the anelasticity is an intrinsic property of CuO NWs although the electron
beam irradiation was proved to be capable of accelerating the shape recovery process. A twin associated atoms
cooperative motion mechanism was proposed to account for this phenomenon. These results provide an insight
of the mechanical properties of CuO NWs which could be promising materials in nanoscale damping systems.
KEYWORDS anelasticity, CuO, nanowires, shape recovery, twin
As the oxide of copper, CuO has been
attracting great research interests since the early
20th century [1], due to the widespread applications
of copper in our daily life. Recently, extensive
studies have been focused on the one-dimensional
(1D) CuO nanowires (NWs) which exhibit
hyper-elasticity [2], good gas sensing and
photoelectric conversion performances [3-5], etc.,
making them promising candidates as active
components in the next generation micro- or
nanoelectromechanical systems (MEMS or NEMS)
[2, 4, 6]. However, since the functions and service
life of all materials are highly dependent on the
mechanical properties, full realization of the CuO
NWs’ potential applications requires a
comprehensive understanding of their mechanical
behavior, which is yet to be explored.
So far, significant progresses have been made in
studying the mechanical properties of 1D NWs,
especially the metallic NWs [7-11] with higher
Nano Res DOI (automatically inserted by the publisher)
Research Article
————————————
Address correspondence to Jianbo Wang, wang@whu.edu.cn
3
strength and improved ductility as compared with
the bulk counterparts [12-14]. In sharp contrast, since
the CuO NWs are inherently brittle at room
temperature, most reports dealt with their elastic
behaviors. Specifically, both the experiments [2, 15,
16] and theoretical calculations [12, 17] indicate that
the Young’s modulus of an individual CuO NW is
dependent on its diameter. Meanwhile, it is
reasonable to assume that the NWs embedded in a
nanodevice may suffer from external mechanical
stresses (e.g., compression, bending and buckling) on
repeated occasions. Nonetheless, little information is
available on the deformation processes of CuO NWs
within a complete loading-unloading cycle.
Herein, applying the in situ transmission
electron microscopy (TEM), the response of a single
CuO NW to external compressive stress is directly
monitored. Interestingly, different from conventional
mechanical properties: elasticity [8, 10] (resume its
original shape instantaneously once the external
stress is unloaded) and plasticity [18, 19] (irreversible
deformation strain commonly associated with the
dislocation behavior in ductile materials), the NWs
show an unexpected anelasticity, as exemplified by a
delay in shape recovery after the release of the
mechanical stress. In addition, it is found that the
electron beam (e-beam) irradiation can expedite the
recovery procedure, signifying that the cooperative
motion of atoms may be responsible for the
anelasticity.
The experiments were mainly performed inside
a JEOL JEM-2010 FEF (UHR) electron microscope
equipped with a Nanofactory EP1000 TEM-scanning
tunneling microscopy platform. Selected area
diffraction (SAED) patterns and some bright/dark
field (BF/DF) images were acquired employing a
JEOL JEM-2010 (HT) electron microscope. Besides, a
Hitachi S-4800 FE-SEM electron microscope was
utilized to take scanning electron microscopy (SEM)
images. Large scale CuO NWs were prepared by
simply heating copper grids at about 400 oC for 3-5
hours in ambient atmosphere. Figure S1 in the
Electronic Supplementary Materials (ESM) shows the
low magnified morphology of the as-fabricated CuO
NWs. Detailed synthesis method can be found in
Refs. [20-22]. Subsequently, the copper grids were cut
into small pieces and attached on a tungsten (W) rod
with conductive epoxy adhesives (Fig. 1(a)).
Simultaneously, a sharp W tip was assembled into
the sample holder which is capable of moving back
and forward in three dimensions (illustrated as x, y, z,
Fig. 1(a)), serving as the other end of the in situ
platform. Figures 1(b)-1(g) illustrate the detailed
procedures of the loading-unloading compressing
mechanical test on an individual NW.
Figure 1 Schematic illustrations of the in situ experiment setup
((a)) and the whole loading-unloading process on an individual
NW ((b)-(g)).
Figure 2(a) is an SEM image showing the
morphology of the as fabricated CuO NWs. The
diameters of the NWs range from tens of nanometers
4
Figure 2 (a) An SEM image of the heated Cu grid surface
covered with CuO NWs. (b) A typical SAED pattern and (c) the
corresponding BF image of a (111) twinned CuO NW. (d)-(e)
DF images obtained by selecting the spots indicated in (b). (f)
The colored superimposed image of (d) and (e).
to several hundred nanometers, while the lengths are
in the range of several hundred nanometers to more
than ten micrometers. Consistent with the previous
reports [20-24], most of the as fabricated CuO NWs
exhibit the monoclinic crystal phase (ICSD
No.016025) with a (1 )11 twinning structure, as
manifested by the SAED pattern presented in Fig.
2(b). Figures 2(c)-2(f) are the corresponding BF and
DF images, which clearly show the existence of the
twin boundary (TB) parallel to the axial growth
direction.
Figures 3(a)-3(c) present the sequential TEM
images of a single CuO NW subjected to the
compressive loading with a displacement rate of
about 6 nm/s. Evidently, the NW gradually
bent/buckled as suggested by the bending contours
(pointed out by arrow heads). Strikingly, the
maximum strain was determined to be as high as
6.18%, corresponding to a stress of 4.32 GPa, given
that the Young’s modulus of CuO is 70 GPa [15]. The
method used to calculate the strain of a bent NW
here is based on the formula = D/2 [18, 25, 26],
where D is the diameter of the NW and is the
curvature radius (see details of calculation in the
ESM). As compared with the bulk materials, such
high strength may result from the lower defect
content as well as the existence of the TB which may
block the dislocation motion [7, 8, 27].
Figure 3 Time-elapsed images showing the deformation of a single CuO NW during the compression ((a)-(c)) and the anelastic
behavior when the stress was unloaded ((d)-(f)). Bending contours are pointed out by the arrow heads. The inset in (c) is the enlarged
view of the dashed squared-area. The NW diameter is approximately 17 nm.
Afterwards, the W tip was retracted back with a
displacement rate of about 37 nm/s (Figs. 3(d)-3(f))
due to the fact that loading speed has an impact on
the shape restoration dynamics. When the external
stress was completely unloaded, a residual bending
strain (0.65%) still existed in the NW (Fig. 3(d)).
Surprisingly, such strain can be gradually
5
annihilated with the elapse of time, representing a
typical anelastic behavior (Detailed deformation
process can be found in Video S1 in the ESM). The
other NW shown in Fig. 3 can serve as a reference
point. Such anelasticity has been consistently found
in dozens of CuO NWs that were tested. The typical
residual strain (ε) versus time (t) curves of four
different NWs after unloading are presented in Fig. 4.
The characteristics of the curves (e.g., the strain
recovery becomes more and more slowly) are similar
to those of materials with reported anelastic behavior
[28-30]. The residual strain of CuO NWs can be
represented in an exponential function form
simplified from Refs. [29, 30]
( ) e ktt C (1)
where C is the maximum residual strain and k is a
parameter characterizing the ease or difficulty in
recovery of materials (k = 0.016 ± 0.008 s-1, 0.007 ±
0.004 s-1, 0.011 ± 0.003 s-1, 0.021 ± 0.005 s-1 for the NWs
in Fig. 4, respectively). It is worthy of noting that the
anelastic behavior in CuO NWs did not show the
obvious dependence on the NW diameters, although
size effects played an important role in the
mechanical behaviors of many 1D NWs [14, 15, 31].
Figure 4 Four ε-t curves showing the anelasticity of CuO NWs.
The diameters of the NWs are 37 nm (in black), 53 nm (in red),
92 nm (in green), and 17 nm (in blue).
To illustrate the e-beam irradiation effect on the
anelastic deformation, another loading-unloading
cycle was performed in the same NW shown in Fig. 5.
It should be noted that, after the stress was released,
the e-beam was turned off immediately and the
images were taken by intermittently turning on the
beam for about 2 seconds per 10 or 15 minutes (Figs.
5(a)-5(f)). An initial residual strain of 0.33% was
obtained (Fig. 5(a)) which again gradually reduced
(Figs. 5(b)-5(f). See also Table S1 in the ESM) without
the assistance of e-beam illumination, indicating that
the anelasticity is an intrinsic property of the NWs.
Furthermore, after the e-beam was turned on, the
residual strain continued to decrease and finally
disappeared (Figs. 5(h)-5(m)), and the NW has
returned to its original shape. The corresponding ε-t
curve is shown in Fig. 5(o). It is evident that the
e-beam irradiation would speed up the recovery
process of the strained CuO NW.
Figure 5 (a)-(n) TEM images showing the shape recovery
process of a CuO NW with ((a)-(f)) and without ((h)-(m))
e-beam irradiation. (g) and (n) present the superimposed
imaged of (a)-(f) and (h)-(m), respectively. (o) The related ε-t
curve based on (a)-(n).
6
The anelasticity had been well documented in
other systems, including metals (metal alloys) and
semiconductors. However, the underlying
mechanisms are different. For metals, anelasticity
was frequently observed in nanocrystalline (NC)
materials, and associated with their large content of
grain boundaries (GBs). For instance, in NC Au, the
large anelastic strain was considered to be caused by
cooperative motion of atoms in the GBs [28, 32].
Regarding the semiconductor materials, the
anelasticity of GaAs NWs originated from the
amorphous surface layer [33], induced by the
inevitable oxidation during the sample preparation.
When the applied external stress was unloaded, the
amorphous layer holds back the crystal core,
resulting in a slow recovery [33]. It is reasonable that
this kind of anelasticity is dependent on the NW
diameter, because the thicker surface amorphous
layer would directly provide a much higher recovery
driving force as compared to that induced by the
thinner one [33]. Other mechanisms such as phase
transition, motion of twins were also proposed to
explain the anelasticity in metal alloys [34, 35]. In the
current case, no deformation-induced twinning
and/or phase transition is seen during the entire
loading-unloading cycle. In addition, as a typical
oxide, the surfaces of the tested NWs are not always
covered by the amorphous layer (Fig. S2 in the ESM),
implying that the anelasticity cannot be attributed to
the surface coating alone. Thus, it is speculated that
the twinning structure may account for the
anelasticity in CuO NWs. Conventionally, twinning
induced pseudoelasticity has been explained by the
motion of atoms in the vicinity of the TB core [35].
Similarly, due to the low activation energy of grain
boundary atoms [28], it’s reasonable to propose that,
under large external bending stress (e.g., 4.3 GPa),
atoms adjacent to the TB would move out from their
original sites to new places (defects gliding along TB
[27, 36]), in order to counteract local lattice distortion.
Once the external stress is removed, those
rearranged atoms tend to move back to their original
sites motivated by the global lattice distortion stress.
However, the relaxation can’t be achieved at once
when the temperature is low, e.g., room temperature
[35], resulting in the anelastic deformation. Besides,
twin boundaries can block the motion of dislocations,
thus preventing the NWs from fracture and facilitate
the anelasticity [9, 27].
Furthermore, there are two major effects of
e-beam irradiation: (1) knock-on displacement:
Basically, the constituent atoms will receive energy
from the incidental electrons. The transferred energy
E is given by [13, 37, 38] 2
max sin ( /2)E E (2) 6
max 0 0(1.02 /10 )/(465.7 )E E E A (3)
where θ is the deflection angle of the electron in the
field of atom nucleus, Emax is the maximum energy
transferred, E0 is the energy of incidental electrons,
and A is the mass number of the irradiated atom (all
the energy here is in eV). In the current situation, the
maximum energies transferred from 200 keV
electrons to Cu atoms and O atoms are 8.2 eV and
32.7 eV, respectively. Although there is no available
data of the displacement energy threshold ET of Cu
and O atoms in CuO, considering the covalent
bonding (bonding energy, 4-8 eV) and ET of similar
materials (e.g., in GaAs, ET Ca = 9 eV, ET As = 9.4 eV; in
CdTe, ET Cd = 5.6 eV, ET Te = 7.9 eV; ET X represents the
ET of X atoms), the displacement is likely to occur
[39]. Moreover, for grain boundary atoms, the ET is
much lower than the bonding energy. It is thus
convenient to claim that the e-beam should expedite
the atomic diffusion at TB and thus accelerate the
strain recovery process [39]. (2) Sample heating: The
temperature rise is estimated to be around 0.4 K (see
calculation details in the ESM), which as well is
theorized to accelerate the strain recovery process
[29]. On the basis of these results, the occurrence of
TB is believed to be the major effect leading to the
anelasticity.
To sum up, an unexpected anelastic behavior
has been observed in CuO NWs. In situ investigation
suggests that the anelasticity is an intrinsic property
of CuO NWs. Meanwhile, e-beam irradiation has
been demonstrated to be capable of expediting the
recovery of NWs. Future studies are of necessity in
view of revealing the detailed atomic process
associated with the anelasticity by means of both
real-time observations and atomistic simulations.
Acknowledgements
This work was supported by the 973 Program (No.
2011CB933300), the National Natural Science
Foundation of China (Nos. 51271134, J1210061), the
7
Fundamental Research Funds for the Central
Universities, the CERS-1-26 (CERS-China Equipment
and Education Resources System), and the China
Postdoctoral Science Foundation (Nos. 2013M540602,
2014T70734).
Electronic Supplementary Material: Supplementary
material (further details of the CuO NWs’
morphology and surface, strain data, in situ
loading-unloading video and the calculation of
e-beam induced temperature rise) is available in the
online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
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