Flexible High-Output Nanogenerator Based on
Lateral ZnO Nanowire Array
Guang Zhu,
†
Rusen Yang,
†
Sihong Wang, and Zhong Lin Wang*
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
ABSTRACT We report here a simple and effective approach, named scalable sweeping-printing-method, for fabricating flexible high-
output nanogenerator (HONG) that can effectively harvesting mechanical energy for driving a small commercial electronic component.
The technique consists of two main steps. In the first step, the vertically aligned ZnO nanowires (NWs) are transferred to a receiving
substrate to form horizontally aligned arrays. Then, parallel stripe type of electrodes are deposited to connect all of the NWs together.
Using a single layer of HONG structure, an open-circuit voltage of up to 2.03 V and a peak output power density of ∼11 mW/cm
3
have
been achieved. The generated electric energy was effectively stored by utilizing capacitors, and it was successfully used to light up a
commercial light-emitting diode (LED), which is a landmark progress toward building self-powered devices by harvesting energy
from the environment. This research opens up the path for practical applications of nanowire-based piezoelectric nanogeneragtors
for self-powered nanosystems.
KEYWORDS Nanogenerator, ZnO, nanowire, light-emitting diode, self-powering
E
nergy harvesting is critical to achieve independent
and sustainable operations of nanodevices, aiming
at building self-powered nanosystems.
1-3
Taking the
forms of irregular air flow/vibration, ultrasonic waves, body
movement, and hydraulic pressure, mechanical energy is
ubiquitously available in our living environment. It covers a
wide range of magnitude and frequency from cell contrac-
tion to ocean waves. The mechanical-electric energy conver-
sion has been demonstrated using piezoelectric cantilever
working at its resonating mode.
4-7
However, the applicabil-
ity and adaptability of the traditional cantilever based energy
harvester is greatly impeded by the large unit size, large
triggering force and specific high resonance frequency.
Recently, a series of rationally designed nanogenerators
(NGs) with piezoelectric nanowires (NWs) have shown great
potentialtoscavengetinyandirregularmechanicalenergy.
8-15
However, insufficient electric output hinders their practical
applications. We report here a simple and effective ap-
proach, named scalable sweeping-printing-method, for fab-
ricating flexible high-output nanogenerator (HONG). An
open-circuit voltage of up to 2.03 V and a peak output power
density of ∼11 mW/cm
3
have been achieved. The generated
electric energy was effectively stored by utilizing capacitors,
and it was successfully used to light up a commercial light-
emitting diode (LED), which is a landmark progress toward
building self-powered devices by harvesting energy from the
environment. Furthermore, by optimizing the density of the
NWs on the substrate and with the use of multilayer integra-
tion, a peak output power density of ∼0.44 mW/cm
2
and
volume density of 1.1 W/cm
3
are predicted.
The mechanism of converting mechanical energy by a
single ZnO NW that is laterally bonded to a substrate has
been discussed in details in our previous report.
13
Owing to
much smaller diameter of the NW compared to the substrate
thickness, outward bending of the substrate induces a
uniaxial tensile strain in the NW. Because of the piezoelectric
property of the ZnO NW, the stress results in a piezoelectric
field along the length, which causes a transient charge flow
in the external circuit. The Schottky contact at the bonded
ends can regulate the charge flow. As a result, the bending
and releasing of the single-wire-NG gives rise to an alternat-
ing flow of the charges in the external circuit. In this work,
the power output has been scaled up with the integration of
hundreds of thousands of horizontally aligned NWs, which
was made by a scalable sweeping-printing-method that is
simple, cost-effective, and highly efficient.
The method consists of two main steps. In the first step,
the vertically aligned NWs are transferred to a receiving
substrate to form horizontally aligned arrays. The major
components of the transfer setup are two stages (Figure 1a).
Stage 1 has a flat surface that faces downward and holds
the vertically aligned NWs; stage 2 has a curved surface and
holds the receiving substrate. Polydimethylsiloxane (PDMS)
film on the surface of stage 2 is used as a cushion layer to
support the receiving substrate and enhances the alignment
of the transferred NWs. The radius of the curved surface of
stage 2 equals the length of the rod supporting the stage,
which is free to move in circular motion (Supporting Infor-
mation Figure S1). In the second step, electrodes are depos-
ited to connect all of the NWs together.
Vertically aligned ZnO NWs on Si substrates were syn-
thesized using physical vapor deposition method.
16,17
The
dense and uniform NWs have the length of ∼50 µm,
diameter of ∼200 nm, and growth direction along the c-axis
* To whom correspondence should be addressed. E-mail: zlwang@gatech.edu.
†
Authors with equal contribution
Received for review: 6/3/2010
Published on Web: 07/21/2010
pubs.acs.org/NanoLett
© 2010 American Chemical Society
3151 DOI: 10.1021/nl101973h | Nano Lett. 2010, 10, 3151–3155
(Figure 1b, Supporting Information Figure S2). The same
growth direction of NWs guarantees the alignment of the
piezoelectric potentials in all of the NWs and a successful
scaling up of the output, which will be elaborated later. A
small piece of Si substrate with grown ZnO NWs was
mounted onto stage 1 (Figure 1a) and a piece of Kapton film
with the thickness of 125 µm was attached to stage 2 (Figure
1a). The distance between the receiving substrate and NWs
was precisely controlled to form a loose contact between the
two. The receiving substrate then counterclockwise swept
across the vertical NWs arrays, which were detached from
Si substrate and aligned on the receiving substrate along the
direction of sweeping due to the applied shear force (Figure
1a). The as-transferred NWs are presented in Figure 2c with
an estimated average density of 1.1 × 10
6
cm
-2
. The length
variation is probably due to the fact that not all of the NWs
were broken off at the roots.
Next, the evenly spaced electrode pattern over the aligned
NWs was first defined using photolithography and then
followed by sputtering 300 nm thick Au film (Figure 1d).
After lifting off the photoresist, 600 rows of stripe-shaped
Au electrodes with 10 µm spacing were fabricated on top of
the horizontal NW arrays (Figure 1e). Au electrodes form
Schottky contacts with the ZnO NWs, which are mandatory
for a working NG.
8,18
Approximately 3.0 × 10
5
NWs in an
effective working area of 1 cm
2
, as pointed by an arrowhead
in Figure 1d (inset), are in contact with electrodes at both
ends. Finally, a PDMS packaging over the entire structure
can further enhance mechanical robustness and protect the
device from invasive chemicals.
The working principle of the HONG is illustrated by the
schematic diagrams in Figure 2a,b. NWs connected in
parallel collectively contribute to the current output; NWs in
different rows connected in serial constructively improve the
voltage output. The same growth direction of all NWs and
the sweeping printing method ensure that the crystal-
lographic orientations of the horizontal NWs are aligned
along the sweeping direction. Consequently, the polarity of
FIGURE 1. Fabrication process and structure characterization of the HONG. (a) Experimental setup for transferring vertically grown ZnO NWs
to a flexible substrate to make horizontally aligned ZnO NW arrays with crystallographic alignment. (b) SEM image of as-grown vertically
aligned ZnO NWs by physical vapor method on Si substrate. (c) SEM image of the as-transferred horizontal ZnO NWs on a flexible substrate.
(d) Process of fabricating Au electrodes on horizontal ZnO NW arrays, which includes photolithography, metallization, and lift-off. (e) SEM
image of ZnO NW arrays bonded by Au electrodes. Inset: demonstration of an as-fabricated HONG. The arrowhead indicates the effective
working area of the HONG.
© 2010 American Chemical Society
3152
DOI: 10.1021/nl101973h | Nano Lett. 2010, 10, 3151-–3155
the induced piezopotential is also aligned, leading to a
macroscopic potential contributed constructively by all of the
NWs (Figure 2b).
To investigate the performance of the HONG, a linear
motor was used to periodically deform the HONG in a cyclic
stretching-releasing agitation (0.33 Hz). The open-circuit
voltage (V
oc
) and the short-circuit current (I
sc
) were measured
with caution to rule out possible artifacts.
19
At a strain of
0.1% and strain rate of 5%s
-1
, peak voltage and current
reached up to 2.03 V and 107 nA, respectively. Assuming
that all of the integrated NWs actively contribute to the
output, the current generated by a single NW is averaged to
be ∼200 pA; and the voltage from each row is ∼3.3 mV in
average. Considering the size of the working area of the
nanogenerator (1 cm
2
) (Figure 1e, inset), a peak output
power density of ∼0.22 µW/cm
2
has been achieved, which
is over 20-fold increase compared to our latest report based
on a more complex design.
14
For nanowires with the
diameter of ∼200 nm, the power volume density is ∼11
mW/cm
3
, which is 12-22 times of that from PZT based
cantilever energy harvester.
6,7
The durability test and further
characterization were performed, which prove the stability
and robustness of the HONGs (Supporting Information
Figure S3). Voltage linear superposition test verified the
proposed working principle of the HONGs (Supporting In-
formation Figure S4).
Further scaling up the power output is expected to be
technically feasible. If NWs can be uniformly and densely
packed as a monolayer over the entire working area, and
all can actively contribute to the output, the maximum
power area density is expected to reach ∼22 µW/cm
2
. The
power volume density is anticipated to be improved up to
∼1.1 W/cm
3
. With 20 layers of such NW arrays stacked
together, the power area density would be boosted up to
∼0.44 mW/cm
2
.
The performance of the HONG is affected by strain and
strain rate. For a given strain rate (5% s
-1
), an increase in
strain leads to a larger output (Figure 3a,b). Likewise, at a
constant strain (0.1%), the output is proportional to the
strain rate (Figure 3c,d). Beyond a certain strain and strain
rate, saturation of the magnitude occurs, probably due to
the converse piezoelectric effect, which is the strain created
by the piezopotential and it is opposite to the externally
induced strain. It is noticed that 0.1% strain is sufficient to
induce effective output, which is much smaller than the 6%
fracture strain of the ZnO NW predicted theoretically.
20
Storing the generated energy and driving functional
devices are extremely important steps toward practical
applications of the nanogenerator. In this work, they were
accomplished by using a charging-discharging circuit with
two consecutive steps (Figure 4). The circuit function is
determined by the status of a switch (Figure 4a inset). The
FIGURE 2. Working principle and output measurement of the HONG. (a) Schematic diagram of HONG’s structure without mechanical
deformation, in which gold is used to form Schottky contacts with the ZnO NW arrays. (b) Demonstration of the output scaling-up when
mechanical deformation is induced, where the “(” signs indicate the polarity of the local piezoelectric potential created in the NWs. (c) Open
circuit voltage measurement of the HONG. (d) Short circuit current measurement of the HONG. The measurement is performed at a strain of
0.1% and strain rate of 5% s
-1
with the deformation frequency of 0.33 Hz. The insets are the enlarged view of the boxed area for one cycle
of deformation.
© 2010 American Chemical Society
3153
DOI: 10.1021/nl101973h | Nano Lett. 2010, 10, 3151-–3155
switch is at position A for energy storage achieved by
charging capacitors. Upon charging completion, the switch
is switched to position B for energy releasing to power a
functional device, such as a light emitting diode.
It is the key for a successful and effect energy storage to
take full advantage of the alternating output. As a result, an
integrated full wave rectifying bridge (Transys Electronics
Limited, DI 102) was connected between a HONG and
FIGURE 3. Performance characterization of the HONG with increasing strain and strain rate. (a) Open circuit voltage measurement of the
HONG with increasing strain at a given strain rate of 5% s
-1
. (b) Short circuit current measurement of the HONG with increasing strain at a
given strain rate of 5% s
-1
. (c) Open circuit voltage measurement of the HONG with increasing strain rate at a constant strain of 0.1%. (d)
Short circuit current measurement of the HONG with increasing strain rate at a constant strain of 0.1%. For all measurements, the mechanical
deformation frequency is fixed at 0.33 Hz.
FIGURE 4. Application of the electric energy generated by the HONG to drive a commercial light emitting diode. (a) The electric output measured
after a full wave rectifying bridge. Signals of negative signs are reversed, as pointed by the arrowhead. Inset: Schematic of the charging-
discharging circuit for storing and releasing the energy generated by the HONG, respectively. (b) Image of a commercial LED, which is
incorporated into the circuit. (c) Image of the LED in dim background before it was lit up. (d) Image of the LED in dim background at the
moment when it was lit up by the energy generated from the HONG.
© 2010 American Chemical Society
3154
DOI: 10.1021/nl101973h | Nano Lett. 2010, 10, 3151-–3155
capacitors (Vishay Sprague, Type 430P, 2 µF ( 10%). The
output of the HONG measured after the bridge exhibits only
positive signals (Figure 4a). Full wave rectification achieved
by the bridge ensures energy storage at an enhanced ef-
ficiency, although the rectified signal (as pointed by an
arrowhead in Figure 4a) has appreciably reduced magnitude
due to the reverse current leakage of the diodes in the bridge;
this reducing effect is rather notable at small output current.
To facilitate the charging process, the output frequency of
the HONG was tuned up to 3 Hz by reducing the periodicity
of the mechanical deformation. Ten capacitors were con-
nected in parallel such that they were simultaneously charged,
and the voltage across a single capacitor finally reached 0.37
V.
Upon finishing charging, the capacitors were reconfigured
from parallel connection to series connection, leading to a
total voltage source of 3.7 V. The stored electricity was used
to drive a commercial red LED (Figure 4b, Avago Technolo-
gies US Inc., HLMP-1700), which has an emission spectrum
centered at 635 nm. The turn-on voltage and forward-biased
resistance are 1.7 V and 450 Ω, respectively. The discharg-
ing process was triggered, leading to a maximum discharg-
ing current of 4.5 mA and the LED was lit up. The emitted
light lasted 0.1-0.2 s and was clearly captured in dim
background (Figure 4c,d, video in Supporting Information).
During the whole charging-discharging process, no other
power sources were involved. The entire circuit is essentially
a complete self-powered system, which consists of three
components: an energy harvester (the HONG), storage units
(capacitors), and a functional device (the LED).
An effective energy generation efficiency is defined as the
ratio between the energy stored by the capacitors and the
strain energy input to all of the active NWs, and it takes into
account the performance of the electronic components in
the circuit. The total electrical energy stored by the capacitor
can be calculated as W
stored
) CU
2
n/2 ) 1.37µJ, where C is
the capacitance of a single capacitor, U is voltage across the
capacitor, and n is the number of capacitors. Since the
dominant strain in the ZnO NWs is tensile strain, with shear
strain safely neglected, the total strain energy can be esti-
mated as W
strain
) πD
2
L
0
Eε
2
ftn
0
/8 ) 30 µJ, where D is the
diameter of the NW (200 nm), L
0
is its original length (10
µm), which is fixed by the electrode spacing, E is the Young’s
modulus (30 GPa), ε is the strain of NWs (0.1%), f is the
frequency of deformation (3 Hz), t is the total charging time
(7200 s), and n
0
is the number of integrated NWs
(300 000).
13
Therefore, the effective energy generation ef
-
ficiency is estimated to be ∼4.6%. This value is naturally
lower than the energy conversion efficiency of a single nano/
microwire (∼7%), which is defined as the ratio between the
generated electric energy (W
generated
) ∫VI dt, where V is the
voltage, and I is the current) and the mechanical input strain
energy.
13
This is mainly attributed to the energy dissipation
on rectifying bridge and capacitors, as elaborated in the
Supporting Information.
In summary, we have successfully fabricated high-output
flexible nanogenerators using a sweeping-printing method.
We managed to transfer vertically grown ZnO NWs to a
flexible substrate and achieved horizontally aligned NW
arrays that have crystallographic alignment, based on which
an innovatively designed HONG was fabricated. The electri-
cal output of the HONG reached a peak voltage of 2.03 V
and current of 107 nA with a peak power density of ∼11
mW/cm
3
, which is 12-22 times of that from PZT-based
cantilever energy harvester. An effective energy generation
efficiency of 4.6% was demonstrated. The electric energy
generated by the HONG was effectively stored by capacitors
and used to light up a commercial LED. Furthermore, by
optimizing the density of the NWs on the substrate and with
the use of multilayer integration, a peak output power
density of ∼0.44 mW/cm
2
and volume density of 1.1 W/cm
3
are redicted. This is a key step that is likely to bring
nanogenerator based self-powering technology into people’s
daily life with potential applications in mobile electronics,
health monitoring, environmental inspection, cargo shipping
tracking system, infrastructure monitoring, and even de-
fense technology.
Acknowledgment. Research supported by NSF (DMS
0706436, CMMI 0403671, ENG/CMMI 112024), DARPA
(Army/AMCOM/REDSTONE AR), BES DOE (DE-FG02-
07ER46394), and DARPA/ARO W911NF-08-1-0249. The
authors thank Cheng Li and Benjamin Hansen for their help
on thin film deposition and electric circuit design, respectively.
Supporting Information Available. Experimental setup,
additional figures, and video. This material is available free
of charge via the Internet at http://pubs.acs.org.
REFERENCES AND NOTES
(1) Wang, Z. L. Sci. Am. 2007, 298, 82–87.
(2) Tian, B. Z.; et al. Nature 2007, 449, 885–890.
(3) Pan, C. F.; et al. Adv. Mater. 2008, 20, 1644–1648.
(4) Hausler, E.; et al. Ferroelectronics 1984, 60, 277–282.
(5) Platt, S. R.; et al. IEEE-ASME T. Mech. 2005, 10, 455–461.
(6) Round, S.; Wright, P. K.; Rabaey, J. Comput. Commun. 2003, 26,
1131–1144.
(7) Shen, D. N.; et al. Sens. Actuators, A 2009, 154, 103–108.
(8) Wang, Z. L.; Song, J. H. Science 2006, 312, 242–246.
(9) Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316,
102–105.
(10) Chang, C.; Tran, V. H.; Wang, J.; Fuh, Y.; Lin, L. Nano Lett. 2010,
10, 726–731.
(11) Qin, Y.; Wang, X. D.; Wang, Z. L. Nature 2008, 451, 809–813.
(12) Qi, Y; et al. Nano Lett. 2010, 10, 524–528.
(13) Yang, R.; Qin, Y.; Dai, L.; Wang, Z. L. Nat. Nanotechnol. 2009, 4,
34–39.
(14) Xu, S.; Qin, Y.; Xu, C.; Wei, Y. G.; Yang, R.; Wang, Z. L. Nat.
Nanotechnol. 2010, 5, 367–273.
(15) Choi, D.; et al. Adv. Mater. 2010, 22, 2187–2192.
(16) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947–1949.
(17) Kuo, T.; et al. Chem. Mater. 2007, 19, 5143–5147.
(18) Polyakov, A. Y.; et al. Appl. Phys. Lett. 2003, 83, 1575–1577.
(19) Yang, R.; Qin, Y.; Li, C.; Dai, L.; Wang, Z. L. Appl. Phys. Lett. 2009,
94, No. 022905.
(20) Agrawal, R.; Peng, B.; Espinosa, H. D. Nano Lett. 2009, 9, 4177–
4183.
© 2010 American Chemical Society
3155
DOI: 10.1021/nl101973h | Nano Lett. 2010, 10, 3151-–3155