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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