# Synthesis and Elastic Characterization of Zinc Oxide Nanowires

Authors: M. P. Manoharan; A. V. Desai; G. Neely; M. A. Haque
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Zinc oxide nanowires, nanobelts, and nanoneedles were synthesized using the vapor-liquid-solid technique. Young's modulus of the nanowires was measured by performing cantilever bending experiments on individual nanowires in situ inside a scanning electron microscope. The nanowires tested had diameters in the range of 200–750 nm. The average Young's modulus, measured to be 40 GPa, is about 30% of that reported at the bulk scale. The experimental results are discussed in light of the pronounced electromechanical coupling due to the piezoelectric nature of the material.


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## Body

## 1. Introduction

Zinc oxide (ZnO) exhibits several unique properties, such as being a semiconductor
and a piezoelectric material [1], and consequently, is
used in a wide variety of sensors and actuators. ZnO nanostructures are being
explored for a wide range of applications in nanoscale devices, such as nanogenerators
[2], gas sensors [3], field emission transistors
[4], nanocantilevers [5], and in biomedical
systems, such as ultrasensitive DNA sequence detectors [6]. Apart from the
technological significance of ZnO nanostructures, their quasi one-dimensional
structure with diameters in the range of tens of nanometers to hundreds of
nanometers makes them interesting from a scientific point of view. In this size
range, they are expected to possess interesting physical properties and pronounced
coupling that are quite different from their bulk counterpart [7].Although
ZnO nanowires are touted as the next generation materials for use in nanoscale
systems [8], very few experimental
investigations on their mechanical properties are reported in literature. The
lack of experimental studies is mainly due to the challenges of material
characterization at the nanoscale, such as (i) specimen manipulation, alignment,
and gripping to achieve the desired boundary conditions, and (ii) application
and measurement of force and displacement with very high resolution [1]. Additionally, ability
to perform in situ experiments is
important for nanoscale materials characterization. In situ   experiments, usually conducted in analytical chambers
such as the scanning or transmission electron microscope (SEM or TEM), enable
direct visualization of the events as they occur, thus providing qualitative
information along with quantitative data. In situ experiments also
ensure accuracy of the experimental procedures, which is challenging to supervise
at the nanoscale.In this paper, we study the effect of temperature and gas flow rate on the growth of
different zinc oxide nanostructures synthesized using the vapor-liquid-solid (VLS)
technique and present experimental results on Young's modulus of single ZnO
nanowires. The modulus was measured by bending the nanowire in a cantilever
configuration inside a SEM, enabled for in situ observations. In
Section2, we review the main techniques in literature for mechanical characterization
at the nanoscale, and experimental results on Young's modulus of zinc oxide
nanostructures.


## 2. Review of Nanomechanical Experimental  Techniques and Young’s Modulus Values  for Zinc Oxide Nanostructures

One of the direct techniques to measure Young's modulus of materials is uniaxial
tensile testing. However, it is difficult to adapt this technique for nanoscale
material characterization, due to the reasons mentioned in Section1. Microelectromechanical
systems (MEMS) are used as test beds for characterizing the mechanical
properties of nanostructures to circumvent some of these problems at the cost
of complexity in device design and fabrication [19–21]. Desai and
Haque [1] have used the uniaxial
tensile testing technique to measure Young's modulus of ZnO nanowires. It is
interesting to note that the authors reported fracture strains as high as 15%
for the nanowires, which is unusual considering that bulk ZnO is a brittle material
at the bulk scale [22]. Young's modulus of
nanowires can also be extracted from the resonant frequency of a single
nanowire, induced by an alternating electric field. As examples, researchers
have used this dynamic characterization technique to measure the modulus of
zinc oxide nanowires [13], carbon nanotubes [23], and gallium nitride
nanowires [24].The quasi-static
counterpart of the dynamic experiments essentially involves bending the
nanowire specimen with a very soft spring (e.g., cantilever beam). The
experiment is generally performed using an Atomic Force Microscope (AFM). Here,
the deformation is primarily strain gradient dominant at the rigid support.
Song et al. [11] and Hoffmann et al. [17] have used this
technique to measure Young's modulus of ZnO nanowires. It is important to note
that the effect of varying levels of strain gradient in these experiments
(caused by bending) may result in significant deviation in the mechanical
properties, because of the piezo-electric nature of the material. AFM-based
experiments are popular techniques for mechanical characterization, because the
stiffness of the tip is very small, and hence, the force measurement resolution
is very high (on the order of nano-Newtons). However, understanding the tip-nanowire interaction is crucial for
accurate and reliable experimental studies. For instance, friction (due to slipping)
and van der Waals forces between the nanowire and tip will introduce errors in
the measurement of mechanical properties [25]. The influence of these
surface forces on the mechanical properties will be more significant in the case
of smaller diameter (less than 30–40 nm) or high aspect ratio nanowires (greater
than 100), where the magnitude of forces is very small (on the order of
pico-Newtons to few nano-Newtons). It is also important to note that specimen geometry,
crystallographic orientation, synthesis process, and the nature of experimental
technique–uniform strain versus strain gradient-dominant
and static versus dynamic deformation, all significantly affect the
experimental results. Consequently, a huge spread is observed in Young's
modulus values reported in literature for zinc oxide nanowires, as summarized
in Table 1.Table 1
Young’s modulus values of ZnO nanostructures reported in literature.
No.Young’s modulus (GPa)Nanostructure typeReferenceTechnique(1)52Nanobelts[9]Mechanical resonance in TEM(2)50Nanobelts[10]TEM resonance(4)29 ± 8Nanowire[11]AFM
bending (vertical nanowires)(5)140–210 (size effect reported)Nanowire[12]SEM resonance(6)58Nanowire[13]TEM resonance(7)90–100Nanowire[14]Nanoindentation(8)38.2 by AFMNanobeltNanoindentation and 3-point AFM bending31.3 by[15]nanoindentation(9)106 ± 25Nanowire[16]Detecting resonance under an optical microscope(10)97 ± 18Nanowire[17]Tensile and cantilever bending experiments, measured modulus only at fracture(11)117, 229 (100 nmNanowireBuckling of nanowires using nanoindentationdiameter)[18]232, 454 (30 nm diameter)In this paper, we present results on in situ cantilever bending experiments inside a
Focused Ion Beam–Scanning Electron Microscope (FIB-SEM) on ZnO
nanowires. These experiments provide information on the elasticity of ZnO
nanowires determined using a quasi-static and
strain gradient-dominated technique. In situ experiments enabled us to observe
the nanowires-tip interaction during the experiment.In Section 3, we discuss the synthesis process
of zinc oxide nanowires.


## 3. Nanowire Synthesis Process

We synthesized the zinc oxide
nanowires by the vapor-liquid-solid (VLS) mechanism [26] using gold as a
catalyst. The Lindberg Anneal single tube furnace (Blue M) was used for the
nanowire growth process; the schematic of the furnace is shown in Figure 1. We
started with ZnO powder (Alfa Aesar, 99.99%) and graphite powder (Alfa Aesar, 99.99%)
in 1 : 1 ratio by weight in an alumina crucible inside the furnace. Argon gas was
allowed to flow in the tube (from right to left in Figure 1) at 10 sccm. The
silicon (Si) substrates with 20 nm gold (Au) films (on [100] silicon surface) were
placed downstream from the crucible, and served as the platform for nanowire
growth.Figure 1
Vapor-liquid-solid synthesis chamber and process description.As the temperature of the crucible increases to approximately 1000°C, the
ZnO powder is reduced by graphite to form zinc (Zn), carbon monoxide (CO), and
carbon dioxide (CO2) vapors. The argon gas carries these vapor-phase
products to the silicon samples placed at different temperatures. Meanwhile, gold
and silicon droplets form a eutectic alloy at each catalyst site. The gaseous
products produced by the reduction reaction adsorb and condense on the alloy
droplets. Subsequently, the ZnO nanowire synthesis reaction is catalyzed by the
Au-Si alloy at solid-liquid interface to form zinc oxide nanowires [27]. The ZnO vapor saturates
the alloy droplet, followed by the nucleation and growth of solid ZnO nanowire,
due to the super saturation of the liquid droplet. Incremental growth of the
nanowire taking place at the droplet interface, constantly pushes the catalyst
upwards until no more zinc vapor is available, or all the gold is used up. We
observed nanowire growth from around 500°C to 900°C, the
diameters of the nanowire ranged from 30 nm to 750 nm and the lengths were up to
100 μm. Some of the nanowires had a gold tip on the end (Figure 2(c)), indicating
VLS mechanism for the growth process. We also observed nanobelt (Figure 2(a)) and
nanoneedles (Figure 2(b)) formation in the lower-temperature regions. In
Section 4, we discuss the sample preparation techniques for mechanical characterization
experiments (cantilever bending) of nanowires.Effect of synthesis temperature on the geometry of
nanostructures: (a) nanobelts at 650–700°C, (b) nanoneedles
at 825–875°C, (c) nanowires at 900–950°C.
(a)(b)(c)


## 4. Specimen Preparation

The ZnO nanowires grown by VLS mechanism generally occurred as clusters, but individual
nanowires are required for the experiments. Individual ZnO nanowires were
picked using a micromanipulator (Creative Devices Inc., NJ, USA) fitted with an
electrochemically
sharpened tungsten probe tip. The nanowires adhere to the probe tip due to both
short- and long-range attractive forces, which we generically term as van der
Waal's forces. The nanowire was then placed on the edge of a chip of silicon
wafer (coated with 100 nm thick gold film to improve imaging in SEM), as shown
in Figure3(a). The nanowire was oriented perpendicular to the edge of the
silicon wafer using the probe tip. We “glued” the end of the nanowire near the
edge of the silicon wafer by platinum deposition using a focused ion beam (FIB)
(FEI Quanta 3D 200 FIB/SEM), as shown in Figure 3(b). The inset in Figure 3(b)
shows the platinum deposition or “glue” on the nanowire near the edge of the
silicon wafer. The microscale version of the pick-and-place technique is time
intensive, but enables us to consistently prepare long nanowire specimens for
the experiments. In Section 5, we discuss the experimental technique and
results on Young's  modulus of ZnO nanowires.Zinc oxide nanowire (a) before and (b) after “gluing” with platinum in FIB-SEM.
Inset in (b) shows platinum deposition on the nanowire near the edge of the
wafer.
(a)(b)


## 5. Experimental Setup and Results

We performed cantilever-bending
experiments to estimate Young's modulus of the nanowires. Bending loads were
applied on the nanowires using an AFM cantilever with a known spring constant.
The AFM cantilever (MikroMasch, CSC12) was mounted on the tungsten probe tip in
the omniprobe (a three-axis piezoelectric actuator in the FIB-SEM) along thex-axis (Figure 4(a), probe is not shown in image). The
SEM image plane is the X-Y plane.(a) Tipless AFM cantilever (b) when mounted on the tungsten tip in the omniprobe.
(a)(b)We then tilted the probe such that the tip face was aligned perpendicular to the
viewing screen (parallel to theZ-axis) (Figure 4(b)). This ensures that the
loading direction is in the desired plane (X-Y plane).We then mounted the nanowire specimen inside the SEM chamber, and rotated the SEM stage
(about theZ axis) to align the longitudinal axis (length) of the nanowire
parallel to the length of the AFM cantilever (Figure 5(a)). This ensures that the
central axis of the nanowire and AFM tip are parallel before loading. We then
tilted the stage (around the X axis) to verify that the nanowire is completely
in the X-Y plane. After ensuring that the nanowire and the AFM tip were
aligned, we performed the cantilever-bending experiment inside the SEM. The
schematic of the bending experiment is shown in Figure 5(a). Note that Figure 5(a)
is not to scale and in reality, the nanowire is thinner than the AFM
cantilever. Figure 5(b) shows the in situ bending experiment inside the SEM.(a) Schematic of nanowire-bending experiment, (b) superimposed
images from the in situ bending experiment inside the SEM showing the specimen and only the tip of the loading
structure and (c) “spring” equivalent of the experimental setup.
(a)(b)(c)The AFM cantilever moves vertically downwards (negativeY direction) to vertically load
the nanowire and the deflection of the AFM cantilever and nanowire tip are the
same. We can estimate Young's  modulus of the nanowire from the deflections of
the AFM cantilever base and nanowire tip.We assume the following conditions for the cantilever bending experiments:(1)
clamped fixed end,(2)
small nanowire tip deflections (valid forynw/l≤0.1,
where ynw and l are the tip deflection and length of
the nanowire, resp.). Based on these assumptions, the normal or tensile bending stress (σ) and the normal
or tensile bending strain (ε) on the nanowire, during the cantilever
bending are given by(1)σ=32ktip(ybase−ynw)lπd3,ε=3ynwd2l2,
where ktip is the stiffness of the AFM cantilever tip, ybase is the displacement of
the AFM cantilever base, ynw is the displacement of the nanowire, d is the diameter of the nanowire, and the stresses and strains
are the maximum values that occur on the outermost diameter of the nanowire at
the clamped end. The deflections of the nanowire and AFM cantilever base are
estimated from processing the SEM images during the loading experiments. Thus,
using the deflection values of the AFM cantilever base and nanowire tip and (1), we can estimate the normal stress and strain on the nanowire, and plot
a stress-strain diagram. The slope of the stress-strain curve (linear fit) is
Young's modulus of the nanowire. We performed cantilever bending experiments
inside the SEM on ZnO nanowires specimens with diameters ranging from 350–750 nm
and did not observe any dependence of Young's modulus on the diameter of
the nanowire. Figure 6 shows a representative stress-strain diagram. Young's
modulus values of the nanowires (five specimens) ranged from 35 GPa to 44 GPa,
which is within the expected error in the experimental data. In some of the bending
experiments, the deflections of the nanowires were large and the expressions
for stress and strain (1) are not accurate. In those cases, the
nonlinear moment curvature differential equation is numerically solved to match
the bending profile of the nanowire to obtain more accurate values of the
stresses and strains (details in [25]).Figure 6
Stress-strain diagram for a ZnO nanowire
specimen.For mechanical measurements, the
boundary condition of the cantilever support is critical for an accurate
estimation of the properties. Typically, the nanowire specimen is clamped with
electron-beam-induced deposition [28, 29] or the
focused ion beam-based platinum deposition (FIB-Pt) [19], which might introduce
ion beam induced stresses [25]. However, our in situ
SEM observations show that for nanoscale bending experiments on specimens
without deposition-based clamping, there is no observable rotation of the
nanowire at the fixed end. This suggests that specimen-substrate adhesion could
be strong enough to work as a clamping mechanism.In
order to study the effect of boundary conditions, we repeated the experiment on
adhesion-clamped specimens prepared using the technique employed by Ding et al. [29]. From the experiments on
adhesion-clamped specimens, we measured  Young's modulus of the nanowires to vary
from 18 GPa to 27 GPa (four specimens), and the range of diameters was from 200
nm to 330 nm. This discrepancy can be attributed largely to the difference in
boundary conditions in the two specimen clamping techniques. If the applied
bending force is comparable to the adhesion and friction forces, the rigid
support boundary condition is no longer valid, as the nanowire has free
boundary conditions on a significant part of its outer surface. In Section 6,
the synthesis process and the experimental results are discussed.


## 6. Discussion

We studied the effects of temperature on the synthesis of ZnO nanostructures using the VLS
technique, and subsequently characterized their elastic properties. We observed
no nanowire growth at gas flow rates higher than 10 sccm implying that the
zinc, carbon monoxide, and carbon dioxide vapors are carried away rapidly from
the substrates and do not have enough time to react at the silicon-gold
interface. In most of the cases, the nanowire growth temperatures were between
500 and 800°C,
which is consistent with the binary phase diagram of gold and zinc [30]. Ideally, the nanowire
growth temperatures should be set between the eutectic temperature (683°C) for
gold and zinc and melting point of zinc (420°C), reaching a maximum of 750°C [31]. However, it should be
noted that the equilibrium phase diagrams at the nanoscale might be different
from bulk and could result in different preferential growth temperatures for
the nanowires. In some cases, we observed the growth of different
nanostructures of zinc oxide such as nanobelts and nanoneedles (Figures 2(a) and
2(b)), similar to observations by other researchers [32, 33].From transmission
electron microscope (TEM) images, the growth direction of the nanowire was
determined to be [0001], and the nanowires had a wurtzite crystal structure
(single crystal) with lattice constants close to those of bulk crystals. At the
bulk scale, Young's modulus of zinc oxide in the [0001] direction is 140 GPa [34], which is significantly
higher than the modulus value reported in this paper. This is commonly
attributed to the surface stress effects in the literature. Due to the lower
coordination number of surface atoms compared to bulk atoms, there exist
intrinsic surface stresses in materials [35, 36], and
the mechanical properties of surfaces are different from bulk. The effects of
surface stresses are significant when the size of the material is on the order
of h0=S/E, where S is the surface elastic constant and E is the modulus of the bulk material [37]. For zinc oxide, the
order of h0 is approximately
a few angstroms, which implies that the surface effects cannot alone explain
the size effects observed in Table 1.One of the reasons for the observed scatter in the modulus values (Table1) is the
difference in experimental techniques used to estimate Young's modulus. The
dynamic experiments performed by Huang et
al. [13], Chen et al. [12], and Zhou et al. [16] are expected to show
slightly higher, unrelaxed (EU)
modulus, whereas in this paper we report modulus values estimated by quasi-static experiments (ER).
In dynamic experiments, the time period of motion of the nanowire is much lower
than the relaxation time, and hence, the modulus values estimated during
dynamic experiments tend to be higher than the values estimated during static
or quasi-static experiments (EU>ER) [38]. Also, the oscillating
electric field applied to the specimen induces charges on the nanowire surface,
which can significantly overestimate the elastic properties [39]. For aspect ratios of
around 100 for copper nanowires, the measured modulus could be 1.5 times the
actual modulus.The modulus value reported in this paper is less than the modulus value estimated by
Feng et al. [14] (90–100 GPa), using nanoindentation. They estimated
the modulus of the nanowires from the hardness values of the nanowires, which
were measured during nanoindentation. In order to estimate the hardness of the
nanowire, they had to make assumptions about the elastic properties of the
nanoindenter tip and the nanowire material. In their experiments, they assumed
that the elastic properties of bulk ZnO are applicable to ZnO nanowires, and
this may have influenced the final estimated modulus value of the ZnO nanowire.
Ni and Li [15] estimated the bending Young's
modulus of ZnO nanobelts as 38.2 GPa and nanoindentation modulus as 31.1 GPa,
which compare favorably with the modulus values reported in this paper.A possible mechanism to explain the reduction in modulus of ZnO nanowire compared
to bulk is the strong electromechanical coupling in zinc oxide. Due to its noncentrosymmetric
wurtzite structure and ionic nature of the interatomic bond, internal electric
fields are induced in ZnO when the material is strained [40, 41]. The
positive sign of the electromechanical coupling coefficient, e33,
along the [0001] direction implies that the induced electric field tends to
reduce the measured modulus of the nanowire. Additional electrical polarization
is introduced in the nanowire during flexural deformation due to the
flexoelectric effect which arises because of the high-strain gradient at the
nanoscale [42]. For piezoelectric
materials with low dielectric constant, such as zinc oxide, quasi-static tests
are not recommended for measurements of elastic constants (Young’s modulus)
because of the uncertainties in electrical boundary conditions [43]. As a result, the
measured modulus values in quasi-static nanomechanical characterization (e.g., the
technique reported in this paper) are influenced by the electromechanical
coupling resulting in Young’s modulus of ZnO nanowire being different from
bulk. Another approach for explanation of reduction in modulus is that the
elastic properties of a material can be described at the atomistic level, where
the bond length, bond energy, and arrangement of atoms influence the overall
elastic behavior of the material [44, 45]. In
case of ZnO, the effective charge (e*) on the zinc-oxygen changes due to
charge redistribution when the material is strained [46]. Since Young's
modulus of the material depends on e*, the modulus of the material
should change at higher strains. The techniques used for measuring the elastic
properties at bulk scale involve negligible strains compared to nanoscale
bending experiments. As a result, the measured modulus values of nanoscale zinc
oxide are different from bulk due to strain-dependent modulus.


## 7. Conclusion

Zinc oxide nanostructures (nanobelts, nanoneedles, and nanowires) were synthesized
using the vapor-liquid-solid technique. Young's modulus of the nanowires was
estimated by bending experiments performed in situ in a scanning electron microscope on individual
nanowires. Young's modulus was measured to be about 40 GPa, which is about 30%
of the modulus value at the bulk scale (140 GPa). It was observed that the
specimen preparation technique influences the boundary conditions, which
affects the measured modulus value. The observed size effect was discussed on
the basis of the pronounced electromechanical coupling and strain gradient at
the nanoscale.



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