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Dynamic Article LinksC<Journal ofMaterials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 18946
www.rsc.org/materials FEATURE ARTICLE
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View Online / Journal Homepage / Table of Contents for this issue
Recent advances in power generation through piezoelectric nanogenerators
Brijesh Kumara and Sang-Woo Kim*ab
Received 2nd July 2011, Accepted 1st September 2011
DOI: 10.1039/c1jm13066h
Piezoelectric nanogenerators are promising for the miniaturization of power packages and self-
powering of nanosystems used in implantable bio-sensing, environmental monitoring, and personal
electronics. This paper reviews the importance of nanogenerators as well as the recent advances in
power generation through nanogenerators with the power generation mechanism. This paper discusses
several research and design efforts that enhanced the power generation to commercialize the
nanogenerators not only in nano-systems, but also to power microelectronic devices, such as drive
a commercial liquid crystal display, light up the commercial light emitting diode and laser diode. This
paper will discuss the future goals that must be achieved to find their way to everyday use.
aSchool of Advanced Materials Science and Engineering, SungkyunkwanUniversity (SKKU), Suwon, 440-746, Republic of Korea. E-mail:[email protected]; Fax: +82 31 290 7381bSKKU Advanced Institute of Nanotechnology (SAINT), Center forHuman Interface Nanotechnology (HINT), SKKU-Samsung GrapheneCenter, Sungkyunkwan University (SKKU), Suwon, 440-746, Republicof Korea
Brijesh Kumar
Brijesh Kumar received his PhD
degree, entitled ‘‘Pulsed Laser
Processing of Semiconductor
Submicron and Nano-
structures’’, from Indian Insti-
tute of Technology, Delhi in
2009 under the supervision of
Prof. R. K. Soni. Presently, he is
working with Professor Sang-
Woo Kim as a Research
Professor at School of Advanced
Materials Science & Engi-
neering, Sungkyunkwan
University (SKKU), S. Korea.
His current research areas are
fabrication of energy harvesting
nanoelectronic devices such as
solar cells, nanogenerators,
hybrid devices, and graphene
based devices.
18946 | J. Mater. Chem., 2011, 21, 18946–18958
1. Introduction
Electrical power is most often generated at power stations by
electromechanical generators through chemical combustion or
nuclear fission, geothermal power and kinetic energy of flowing
water. In recent years, with the surge of wireless micro-
electromechanical systems and nanoelectromechanical system
devices, there is increasing demand for clean and efficient power
generation for the self-powering of these devices from ambient
energy sources, such as thermal gradient, solar, mechanical
Sang-Woo Kim
Sang-Woo Kim is an Associate
Professor in School of Advanced
Materials Science and Engi-
neering at Sungkyunkwan
University (SKKU). He
received a PhD from Kyoto
University in Department of
Electronic Science and Engi-
neering in 2004 under the
supervision of Prof. S. Fujita in
the field of wide bandgap
compound semiconductors.
After working as a postdoctoral
researcher at Kyoto University
and University of Cambridge, he
spent 4 years as an Assistant
Professor at Kumoh National Institute of Technology. He joined
the School of AdvancedMaterials Science and Engineering, SKKU
Advanced Institute of Nanotechnology (SAINT) at SKKU in
2009. His recent research interest is focused on piezoelectric
nanogenerators, organic solar cells, optoelectronic devices, and
two-dimensional nanomaterials including graphene. He is the
author of more than 100 peer-reviewed publications.
This journal is ª The Royal Society of Chemistry 2011
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vibration, and bio-fluid. Piezoelectricity, i.e., the conversion of
mechanical energy to electrical signals, is one of the most
versatile phenomena to power small scale electronic devices from
the device environment. In particular, the piezoelectric method
for power generation from harvesting mechanical energy, such as
the body movement, muscle stretching, acoustic/ultrasonic wave,
etc., has attracted a great deal of attention for self-power/wireless
charging, and controllability of the output power.1–6 Power
generation through ambient energy harvesting has several
potentials, such as in sensor network devices that observe an
environment and assemble useful data about the environment.
These are employed in situations where human interactions are
impossible. Hundreds, even thousands of tiny devices should be
placed in some locations, such as an office building or the ocean
floor, or even within a living organism, to monitor certain vari-
ables. Depending on the situations in which these networks are
placed, supplying power for these devices might be an incredibly
difficult task.
Piezoelectric nanogenerators are very promising and offer the
possibility of performing this incredible task of supplying power
for these wireless devices. Recent advances in piezoelectric
nanogenerators open many doors for power generation through
ambient energy harvesting for practical applications.3,7–12 The
use of piezoelectric nanogenerators to capitalize on the vibra-
tions surrounding the device is one method that has observed
a dramatic increase in use for power generation. The active
materials in piezoelectric nanogenerator have crystalline struc-
tures with the ability to effectively transform mechanical strain
energy into electrical charges. This property gives these active
materials the ability to absorb even very minute mechanical
energy from their surroundings, usually ambient vibration, and
transform it into electrical signals that can be used to power other
devices.6,9,13 This paper discusses the recent advances in power
generation through piezoelectric nanogenerators as well as the
future goals that must be achieved to find their way into everyday
use.
2. Importance of piezoelectric nanogenerators
In nanoscience and nanotechnology, developing a novel wireless
nano-scale system, i.e. the integration of nanodevices, functional
components and the power source, is of critical importance for
real-time and implantable bio-sensing, environmental moni-
toring and portable electronics.14–16 These wireless nano-systems
require their own power sources despite their small size and low
power consumption. There are two ways of achieving wireless
nano-systems. One is to use a battery. Even if the battery has
huge capacitance, it has a limited lifetime, and miniaturization of
devices limits the size of the battery, resulting in short battery
lifetime. Therefore, the main challenge relies on the long-lifetime,
small-sized and possibly lightweight batteries. In addition, the
battery must be recharged occasionally. Consequently, the
miniaturization of a power package and self-powering of these
nanosystems are some key challenges for their possible applica-
tions. For biomedical applications, it is important to consider the
toxicity of the materials that compose batteries. The other
approach is to generate electrical power through harvesting the
ambient energies.17 Energy harvesting from the ambient for
powering a nanosystem is very important for its independent,
This journal is ª The Royal Society of Chemistry 2011
wireless and sustainable operation. A piezoelectric nano-
generator is a promising approach for this application.
Energy harvesting in our living environment is a feasible
approach for powering micro-/nanodevices and mobile elec-
tronics due to their small size, lower power consumption, and
special working environment. Nanomaterials have unique
advantages for energy conversion, including solar cells, piezo-
electric nanogenerators, thermoelectric cells, etc.18,19 The type of
energy harvested depends on the applications. For mobile,
implantable and personal electronics, solar energy may not be
the best choice because it is not available in many cases when
which the devices are used. Alternatively, mechanical energy,
including vibrations, air flow, and human physical motion, is
available almost everywhere at all times, which is called random
energy with irregular amplitudes and frequencies. Piezoelectric
nanogeneration is a novel technology that has been developed for
harvesting this type of energy using piezoelectric nanostructure
arrays.
Nanogenerators can be used in areas that require a foldable or
flexible power source, such as implanted biosensors in muscle or
joints, and have the potential of directly converting biome-
chanical or hydraulic energy in the human body, such as flow of
body fluid, blood flow, heartbeat, and contraction of the blood
vessels, muscle stretching or eye blinking, into electricity to
power the body-implanted devices.1,5,6,9 Heart beat-driven flex-
ible nanogenerators can serve as ultrasensitive sensors for real-
time monitoring of the human-heart behavior, which might be
applied to medical diagnostics as sensors and measurement tools,
and confirming the feasibility of power conversion inside a bio-
fluid for self-powering implantable and wireless nanodevices and
nanosystems in a biofluid and any other type of liquid.6 Nano-
generators convert the sound (noise or speech, and even music)
that always exists in everyday life and the environment into
electrical power.3 Nanogenerators would be viable candidates to
meet the world’s energy demands and efforts are continued not
only for powering nanosystems but also for powering micro-/
nano-electronic devices. Strong enough electrical power gener-
ated through nanogenerators has been used to continuously
drive a commercial liquid crystal display (LCD),11 light up
a commercial light-emitting diode (LED)12 and laser diode (LD)8
that confirm the feasibility of using nanogenerators for powering
mobile and personal electronics.
3. Power generation behavior in piezoelectricnanogenerators
Wang and Song first introduced piezoelectric nanogeneration by
examining the piezoelectric properties of a single ZnO nanowire
(NW) by atomic force microscopy in 2006.19 Since then, piezo-
electric semiconductor materials, such as zinc oxide (ZnO),2–20
cadmium sulfide,21,22 zinc sulfide (ZnS),23 gallium nitride
(GaN),24,25 and indium nitride,26 and piezoelectric insulator
materials, such as polyvinylidene fluoride (PVDF),27 lead zirco-
nate titanate (PZT),28,29 and barium titanate,7 have been used in
power generation. The power generation behavior of nano-
generators fabricated with piezoelectric insulating materials
depends mainly on their piezoelectric properties and produces
alternating current (AC) power. The insulting properties of these
materials do not permit carriers to transport from the metal
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electrodes of nanogenerators into these insulating active mate-
rials. As a result, AC power is produced from the nanogenerator.
The corresponding positive and negative voltage, and current
output peaks can be recorded when the active material is
stretched and released repeatedly or the active material experi-
ences strain, which is released repeatedly.27
The mechanism of the power generation behavior of nano-
generators fabricated from piezoelectric semiconductor materials
relies on the coupled semiconducting and piezoelectric properties
and is composed of two processes, which can produce AC power
and direct current (DC) power. Power generation from piezo-
electric semiconductor nanomaterial-based nanogenerators var-
ies with the exerted force directions: perpendicular and parallel to
the axis of the NW, and can be explained as AC and DC power
generation.
Fig. 1 AC power generation mechanism in vertically aligned ZnO NRs-
based nanogenerators. (a) The as-received vertically aligned ZnO NRs-
based nanogenerator. (b) Electrons flow from the top electrode
contacting NRs with a negative potential side to the bottom electrode
contacting the NRs with positive potential through the external circuit
under a compressive force. (c) The piezopotential-induced electrons are
then moved via the external circuit and accumulate at the interface
between the bottom electrode and NRs with positive potential side. (d)
As the external force is removed, the piezoelectric potential inside the
NRs disappears and the accumulated electrons flow back via an external
circuit. Reproduced with the permission of ref. 30.
3.1 AC power generation from piezoelectric semiconductor
nanomaterials
AC power from a nanogenerator is due to the force exerted
perpendicular to the vertically grown piezoelectric semi-
conducting NWs stacked between the bottom and top electrodes.
A Schottky contact must be at least at one end-electrode that
serves as a ‘‘gate’’ and prevents the flow of electrons in the
external circuit through the NW so that the piezoelectric
potential is preserved.3,30 The NW acts as a capacitor and
a charge pump source, which drives the back and forth flow of
electrons in the external circuit when vertically aligned NWs are
pushed and released or lateral NWs on flexible substrates are
stretched and released.3,31 The good quality effective Schottky
contacts at the top and bottom electrodes are preferred for AC
power generation. When piezoelectric semiconducting NWs are
subjected to an external force perpendicular to NWs, a piezo-
electric potential is generated along the NW owing to the relative
displacement of cations with respect to anions under uniaxial
strain. One side of the NW or the nanorod (NR) is subjected to
a negative piezoelectric potential and the other side to a positive
potential. The maximum piezoelectric potential can be calculated
using the equation:32
Vmax ¼ Fg33L/A (1)
where F is the force applied to the top electrode, or the force
applied in stretching the NW, g33 is the piezoelectric voltage
coefficient of the NWs, L is the length of the NWs and A is the
contact area of the NW. The generated negative piezoelectric
potential in the NW must be sufficient to drive the piezoelectric
induced electrons from the top electrode to the bottom electrode
through an external circuit. Fig. 1 shows the proposed typical AC
power generation mechanism. Under uniaxial strain, piezopo-
tential-driven electrons flow from the electrode contacting the
piezoelectric semiconductor NRs or NWs with the negative
potential side to the opposite electrode contacting the piezo-
electric semiconductor NRs or NWs with positive potential
through the external circuit, and accumulated at the interface
with a positive potential side. As the external force is removed,
the piezoelectric potential inside the NRs or NWs disappears and
accumulated electrons flow back via an external circuit.30
18948 | J. Mater. Chem., 2011, 21, 18946–18958
Consequently, an AC voltage and current pulses are recorded by
the pushing force or stretching is removed or released.30,31
3.2 DC power generation from piezoelectric semiconductor
nanomaterials
DC power from the nanogenerator is attributed to the force
exerted perpendicular to the axis of the semiconducting NW; as
a result, the NW bends laterally.2,19 When a NW is subjected to
an external force by the moving tip, deformation occurs
throughout the NW, a piezoelectric potential is generated along
the width of NW owing to the relative displacement of the
cations with respect to anions. The stretched part with the
positive strain will exhibit a positive electrical potential, whereas
the compressed part with the negative strain will show a negative
electrical potential. As a result, the tip of the NW will have an
electrical potential distribution on its surface, whereas the
bottom of the NW is neutralized because it is grounded. The
maximum voltage generated in the NW can be calculated using
the following equation,33,34
Vmax ¼ � 3
4ðk0 þ kÞ�e33 � 2ð1þ yÞe15 � 2ye31
� a3
l3ymax (2)
where k0 is the permittivity in a vacuum, k is the dielectric
constant, e33, e15 and e31 are the piezoelectric coefficients, n is
This journal is ª The Royal Society of Chemistry 2011
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the Poisson ratio, a is the radius of the NW, l is the length
of the NW and nmax is the maximum deflection of the
NW’s tip.
The electrical contact plays an important role in pumping out
charges on the surface of the tips. The effective Schottky contact
must be formed between the counter electrode and the tip of the
NW because the ohmic contact will neutralize the electrical field
generated at the tips. Owing to formation of the Schottky
contact, the electrons will pass to the counter electrode from the
surface of the tip when the counter electrode is in contact with the
region of the negative potential and the current will be measured,
whereas no current will be generated when it is in contact with
the regions of a positive potential, resulting in the generation of
the DC output.2
Wang et al. reported the first ZnO NW-based DC power
nanogenerator based on this principle that was driven by an
ultrasonic wave.2 They used zigzag trenches as the top electrode
to replace the atomic force microscopy (AFM) tip. When it is
subjected to the excitation of an ultrasonic wave, the zigzag
electrode can move down and push the NW. This leads to lateral
bending, which creates a strain field across the NW’s width.
Hence, the NW’s outer surface is in tensile strain and its inner
surface in compressive strain. When the electrode contacts the
NW’s stretched surface, which has a positive piezoelectric
potential, the platinum (Pt) metal–ZnO semiconductor interface
is a reversely biased Schottky diode, resulting in little current
flowing across the interface (Fig. 2b). This process creates,
separates, preserves and accumulates charges. After further
pushing the electrode, the bent NW will reach the other side of
the zigzag electrode’s adjacent tooth. In such a case, the electrode
is also in contact with the compressed side of the NWs, where the
metal–semiconductor interface is a forward-biased Schottky
barrier, resulting in a sudden increase in the output electric
current flowing from the top electrode into the NW. This is the
discharge process. This design works provided there is a relative
displacement between the electrode and NWs. Recently, our
group reported the AC and DC mode control by integrating
nanogenerators with vertical and tilted NRs.30 DC power could
be generated through the nanogenerator with a graphene
Fig. 2 (a) Schematic diagram of the direct current nanogenerator built
using aligned ZnO NW arrays with a zigzag top electrode. An ultrasonic
vibration drives the nanogenerator. (b) An illustration of the zigzag
electrode along with its contact with various NW configurations and the
resulting current. Reproduced with the permission from ref. 35.
This journal is ª The Royal Society of Chemistry 2011
electrode-based ZnO NWs–nanowall hybrid structure with a top
gold (Au) electrode.36
4. Designs and fabrication
To eliminate the use of the AFM tip as reported in the first
nanogenerator for independent operation and technological
applications, there have been various innovation designs for
improving the performance and applicability of the nano-
generators. The first independent operation of a nanogenerator
was also realized by Wang et al. through the design of a nano-
generator with zigzag trenches as a top electrode to replace the
AFM tip, where zigzag trenches act as an array of aligned AFM
tips2 (Fig. 2). This was the DC power nanogenerator and was
demonstrated for an ultrasonic wave with a frequency of 41 kHz.
This work formed the basic platform for optimizing and
improving the performance of the nanogenerators by integrating
them into layered structures. Since then, several vertical NWs-
integrated nanogenerators and lateral NWs-integrated nano-
generators were fabricated by integrating them into layered
structures to improve their performance.5,31,37,38 In addition,
there have been continuing efforts to improve the design and
fabrication of nanogenerators for several technological applica-
tions with better performance.
Flexible nanogenerators
A nanogenerator that is capable of harvesting biomechanical
energy needs to be operated at low frequencies (<10 Hz). The
substrate used for building such nanogenerators has to be flexible
and foldable so that it can respond to the low frequency excita-
tion. Flexible nanogenerators are useful in areas that require
a foldable or flexible power source, such as implanted biosensors
in the muscle or joint, and have the potential of directly con-
verting biomechanical or hydraulic energy in the human body,
such as flow of body fluid, blood flow, heartbeat, and contraction
of blood vessels, muscle stretching or eye blinking, into electricity
to power body-implanted devices. To meet the requirements,
Wang et al. fabricated nanogenerators using a fiber as
a substrate, onto which piezoelectric ZnO NWs were grown
radially around the textile fiber.20 The design is based on two
inter-twisted fibers that form a pair of ‘‘teeth–teeth brushes’’,
with one fiber covered with Au coated NWs and the other just
with bare NWs (Fig. 3). The relative brushing of the NWs rooted
at the two fibers produces electricity via a coupled piezoelectric
semiconductor process. This was the first demonstration of
a fiber-based nanogenerator.
Although the output performance from the fiber-based
nanogenerator was low, this study established the fundamental
methodology for scavenging the body movement energy using
fabric-based nanomaterials. In addition, to improve the output
performance and applicability of the flexible nanogenerators, it is
important to detect simultaneously all forces, pressures and
vibrations in electromechanical systems. In particular, flexibility
and transparency are significant for artificial skins or touch
sensor applications, such as full touch screens and deformable
displays. Therefore, our group reported an integrated trans-
parent flexible nanogenerator with piezoelectric ZnO NRs that is
suitable for use as a transparent flexible self-powered pressure
J. Mater. Chem., 2011, 21, 18946–18958 | 18949
Fig. 3 (a) Design of the fiber-based two-brush nanogenerator; (b)
optical micrograph of a pair of entangled fibers, one of which is coated
with Au (in darker contrast); (c) the output voltage from the two-fiber
nanogenerator under the pulling and releasing of the top fiber by an
external force. Reproduced with the permission from ref. 20 and 33.
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sensor, which was driven by a mechanical force.32,39 Fig. 4 shows
a schematic diagram of an integrated transparent flexible nano-
generator with piezoelectric ZnO NRs and a field-emission
scanning electron microscopy (FE-SEM) image of ZnO-NR
arrays grown on a flexible indium tin oxide (ITO)-coated poly-
ether sulfone (PES) substrate. The top electrode was placed
above the ZnO NR arrays. The integrated device was then sealed
at the edges to prevent physical and chemical damage. Since the
ZnO NRs and top electrode can be fabricated on flexible
Fig. 4 Schematic diagram of an integrated transparent flexible power
generator and FE-SEM image of ZnO NR arrays on a flexible ITO/PES
substrate (scale bar is 300 nm). Reproduced with the permission of ref. 32.
18950 | J. Mater. Chem., 2011, 21, 18946–18958
substrates, it is possible for the integrated nanodevice to be fully
flexible. Furthermore, the device can be transparent, depending
on the materials of the top electrode because controlled ZnO-NR
arrays are transparent (over 90%). This device can generate
a current via the deformation of piezoelectric ZnO NRs by
external mechanical forces. This transparent flexible nano-
generator device produces a direct current due to the flexibility of
the top and a bottom electrode, vertical compressive force bends
NRs, and a piezoelectric potential is generated along the width of
the NRs in lateral bending, as shown in Fig. 5. The measured
output current density from this nanogenerator was approxi-
mately 3.7 mA cm�2 under a compressive force of 0.9 kgf. The
compressive force of 0.9 kgf could not fracture the NRs, which
confirms that transparent flexible piezoelectric nanodevices can
act as a reliable nanosystem for applications, such as touch
sensors and artificial skin.
Fully rollable transparent nanogenerator
Transparent metallic or metal oxide films used for transparent
conductors have limited use in flexible electronics due to their
mechanical brittleness, chemical instability and high cost (they
often include noble or rare metals).40,41 Transparent flexible
nanogenerators designed using these metallic or metal oxide film
electrodes, such as ITO, have limited flexibility due to the
ceramic structure of the ITO, and defects can be introduced
easily if the device is overflexed.42,43 Two-dimensional (2D) gra-
phene sheets with extraordinary electrical and mechanical
properties have extremely high mobility (as high as 26 000 cm2
V�1 s�1) at room temperature44 and high mechanical elasticity
(elastic modulus of approximately 1 TPa)45 based on carbon–
carbon covalent bonds, which are favorable for unique applica-
tions in the fields of nanoelectronics. Therefore, a fully rollable
transparent piezoelectric nanogenerator was realized using
chemical vapor deposition (CVD)-grown large-scale graphene
sheets as transparent electrodes in the previous work.46 Fig. 6
presents a schematic diagram of an integrated fully rollable
piezoelectric nanogenerator. To complete the integrated
Fig. 5 Current-density profile generated from the fully flexible piezo-
electric nanodevice under a periodically applied constant compressive
force of 0.9 kgf. Reproduced with the permission of ref. 32.
This journal is ª The Royal Society of Chemistry 2011
Fig. 6 Graphene-based fully rollable transparent nanogenerator. (a)
Graphene sheet prepared by CVD on a nickel-coated SiO2/Si wafer. (b)
Transparent graphene sheet transferred to a flexible polymer substrate.
(c) The ZnO NRs on the transferred graphene sheet (3D heterogeneous
nanostructure), which were grown using an aqueous solution method. (d)
Integrated fully rollable graphene-based nanogenerator. Reproduced
with the permission of ref. 46.
Fig. 7 (a) Current density generated by the graphene-based rollable
transparent nanogenerator and switching-polarity tests. (b) Output
current density before (black line) and after (red line) rolling of the gra-
phene-based nanogenerator. Reproduced with the permission of ref. 46.
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graphene-based nanogenerator, a heterogeneous 3D nano-
structure consisting of 1D ZnO NRs on a 2D graphene electrode
was first prepared. A graphene-based nanogenerator was
completed by integrating the 3D heterogeneous nanostructure
with another graphene sheet as a top electrode, as shown in
Fig. 6d.
The current was measured from the graphene-based nano-
generator by applying the pushing force (1 kgf) to the top of the
nanogenerator in the vertical direction. Fig. 7 shows the current
density generated by the graphene-based nanogenerators. The
output current density was approximately 2 mA cm�2. Despite
the high sheet resistance of the graphene top electrode (�200 U,
which is much larger than that of the commercially available ITO
on plastic substrates, 60–80 U), the output currents were detected
clearly. Moreover, the output current peaks were sharp and
narrow. ‘‘Switching-polarity’’ tests and ‘‘linear superposition’’
tests were performed to confirm that the measured signal was
generated by the graphene-based nanogenerators rather than the
measurement system. When the current meter was forward
connected to a graphene-based nanogenerator, a positive current
pulse was recorded during pushing (before 50 s in Fig. 7a). The
current pulses were also reversed when the current meter was
reverse connected (after 50 s in Fig. 7a). The output current
This journal is ª The Royal Society of Chemistry 2011
density for both connecting conditions was similar. In the linear
superposition tests, the output current of the nanogenerators was
enhanced by connecting them in parallel. The output current was
approximately the sum of the output currents of the individual
nanogenerators. The current output level could be improved by
increasing the work function and reducing the resistance of the
graphene electrodes via the controlled doping process.47
In this study, it was observed that many line defects were
visible to the human eye on the ITO electrodes after rolling ITO-
based nanogenerators on an 8 mm diameter pen. On the other
hand, defects were not visible on the graphene-based nano-
generators after rolling on the same pen. In particular, there were
no differences in the output current measured from the graphene-
based nanogenerator before and after the device had been rolled
several times on the pen (Fig. 7b). A stable and reliable output
current was due to the mechanical and structural strength of the
graphene-based nanogenerators and the electrical stability of the
graphene sheet. The authors examined the in situ electrical
stability of the transferred graphene sheet of 75% transmittance
at a wavelength of 550 nm under mechanical bending. The two-
probe resistance of the graphene sheets after bending was
recovered perfectly. The stable electrical characteristic of a gra-
phene electrode under the bending tests was attributed to the
high mechanical strength and thin thickness of the graphene
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electrode.41,48–50 Therefore, this work highlights the tremendous
potential for the use of graphene sheets as fully rollable trans-
parent electrodes in various flexible electronics. The authors
analyzed the stress distributions for the graphene-based nano-
generator via simulation to clearly understand the mechanical
and structural stabilities of graphene-based nanogenerators
under rolling.
Fig. 8 (a) Schematic diagram of an integrated sound-driven nano-
generator. (b) FE-SEM image of ZnO NW arrays on a GaN/sapphire
substrate. (c) The input signal for the generation of a sound wave and the
output voltage generated from the sound-driven nanogenerator. Repro-
duced with the permission of ref. 3.
Sound-driven nanogenerator
The sound (noise, speech or music) always exists in everyday life
and the environment has been overlooked as a source for
piezoelectric power generation despite the fact that it is a form of
mechanical energy. There should be a way to convert sound
energy from speech, music or noise into electrical power, so that
sound can be used for various novel applications including
mobile phones that can be charged during conversations and
sound-insulating walls near highways that generate electricity
from the sound of passing vehicles. This development would
have the additional benefit of reducing the noise levels near
highways by absorbing the sound energy of vehicles. Fig. 8a and
b shows a schematic diagram of an integrated sound driven
nanogenerator with piezoelectric ZnONWs and a cross-sectional
FE-SEM image of vertically well-aligned ZnONW arrays (acting
as a piezoelectric active layer), respectively.3 The NWs were
grown by thermal CVD via a vapor–liquid–solid mechanism on
an n-type GaN thin film (acting as a bottom electrode)-deposited
sapphire substrate. A PdAu-coated PES substrate was used as
both the top electrode and vibration plate and was placed above
the ZnO NW arrays. The integrated device was then sealed at the
edges to prevent physical and chemical damage. The mean length
and diameter of the ZnO NWs were approximately 10 mm and
150 nm, respectively. The integrated nanogenerator was then
connected to a measurement system.
Fig. 8c shows the output voltage obtained from the integrated
nanogenerator in response to the input signal of the sound wave.
The converted electrical energy from the sound wave was dis-
played on the oscilloscope as a voltage in AC mode, according to
the frequency of the sound wave with the sinusoidal mode elec-
trical input with a small phase difference. This phase difference
between signals was attributed to the impedance of the intrinsic
capacitance and reactance within the piezoelectric circuit. The
intensity of the input sound was 100 decibels (dB) (10�2 Wm�2 at
100 Hz), and the amplitude of the output voltage was approxi-
mately 50 mV. The AC voltage was measured as a result of sound
wave directly transferred to the vertically aligned ZnO NWs,
causing the compressing and release of the NWs. In this work,
the authors examined the voltage generation behavior with the
change in sound wave intensity and frequency. In the linear
superposition test, the output voltages of the nanogenerators
were enhanced by connecting them in series.
1.6 V nanogenerator for mechanical energy harvesting using
PZT nanofibers
PZT is a widely used piezoelectric ceramic material with high
piezoelectric voltage and dielectric constants, which are the ideal
properties of active materials for mechanical to electrical energy
conversion. The piezoelectric voltage constant of the
18952 | J. Mater. Chem., 2011, 21, 18946–18958
semiconducting piezoelectric NWs in the recently reported
piezoelectric nanogenerators was lower than that of PZT nano-
materials, as a result of the lower voltage generation from sem-
iconducting piezoelectric NWs-based nanogenerator. There have
been several successful demonstrations of nanogenerators,
output voltage and power still further to improve for practical
applications. To generate a high output voltage up to 1.6 V, Chen
et al. reported the fabrication of a nanogenerator for mechanical
energy harvesting using PZT nanofibers.28 A highly efficient
nanogenerator based on laterally aligned PZT nanofibers on
interdigitated electrodes was fabricated. The nanogenerator
device was fabricated by the deposition of PZT nanofibers that
had been prepared by electrospinning on the interdigitated
electrodes of Pt fine wire (diameter of 50 mm) arrays, which were
assembled on a Si substrate (Fig. 9a). The diameters of the PZT
nanofibers were controlled to be approximately 60 nm (Fig. 9b)
by varying the concentration of poly vinyl pyrrolidone in the
This journal is ª The Royal Society of Chemistry 2011
Fig. 9 (a) Schematic diagram of the PZT nanofiber generator. (b) SEM
image of the PZT nanofibers across the interdigitated electrodes. (c)
Cross-sectional SEM image of the PZT nanofibers in the PDMS matrix.
(d) Cross-sectional view of the polled PZT nanofiber in the generator. (e)
Schematic view explaining the power output mechanism of the PZT
nanofibers working in the longitudinal mode. The color represents the
stress level in PDMS due to the application of pressure on the top surface.
Reproduced with the permission of ref. 28.
Fig. 10 (a) Measured output voltage from the PZT nanofiber generator
when a small Teflon stack was used to impart an impulsive load to the top
of the PZT nanofiber generator, the inset shows a schematic diagram of
a Teflon stack tapping on the nanogenerator. (b) Output voltage
measured when using a finger to apply a dynamic load to the top of the
generator. The inset shows a schematic diagram of a finger applying the
dynamic load. Reproduced with the permission of ref. 28.
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modified sol–gel solution. The PZT nanofibers were continuous,
whereas the distance between the two adjacent electrodes was 500
mm, as designed. A pure perovskite phase was obtained by
annealing at 650 �C for approximately 25 min. Subsequently,
a soft polymer (polydimethylsiloxane, PDMS) was applied to the
top of the PZT nanofibers (Fig. 9c). The interdigitated electrodes
of fine Pt wires were connected by extraction electrodes to
transport the harvested electrons to an external circuit. Finally,
PZT nanofibers were polled by applying an electric field of 4 �106 V m�1 across the electrodes (Fig. 9d) at a temperature
>140 �C for approximately 24 h. The nanogenerator can be
released from the Si substrate or prepared on flexible substrates.
Fig. 9d and e shows the nanogenerator device and power
generation mechanism, in which PZT nanofibers were working in
This journal is ª The Royal Society of Chemistry 2011
longitudinal mode with an alternating pressure applied to the top
surface of the nanogenerator. The applied pressure was trans-
ferred to the PZT nanofibers. The measured output voltage and
power under periodic stress application to the soft polymer were
1.63 V and 0.03 mW, respectively. The output voltage from the
PZT nanofiber generator was measured when it underwent an
impulsive load, which was applied by tapping the top of the
generator with a small Teflon stack. As shown in Fig. 10a, the
generated voltage, which was induced by piezopotential-driven
transient flow of electrons under an external load, reached 600
mV when a larger impact was applied to the nanogenerator by
periodic knocking. The output voltage generated by the device
increased with increasing impact energy applied to the surface. A
damping effect of the soft polymer matrix on the resonant
J. Mater. Chem., 2011, 21, 18946–18958 | 18953
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frequency was also observed during the energy harvesting
process. In the second application, fingers were used to apply
a periodic dynamic load to the top of the nanogenerator, in
which the positive and negative voltage outputs were observed
(Fig. 10b). A negative voltage distribution was generated due to
the reverse-flowing carriers when the external load was removed
and the piezopotential vanished. The highest output voltage
recorded during the test was 1.63 V. The amplitudes of the
voltage outputs depended on how much pressure had been
applied to the nanogenerator surface.
Flexible high-output nanogenerator based on lateral ZnO NW
arrays
In the case of insulating piezoelectric materials, such as PVDF
and PZT, the reported output voltage was up to 1.6 V and the
output power was 0.03 mW.28 The realization of self-powered
efficient devices with higher output power is still a critical
challenge. Zhu et al. reported the fabrication of flexible high
output nanogenerators with a further improved output voltage
up to 2.03 V and a peak power density of 11 mW cm�3.
Furthermore, they predicted that a peak output power density
of �0.44 mW cm�2,12 and volume density of 1.1 W cm�3 could
be achieved by optimizing the density of the NWs on the
substrate and using multilayer integration. The generated
Fig. 11 Fabrication process and structure characterization of the flexible n
transferring vertically grown ZnO NWs to a flexible substrate. (b) FE-SEM im
by a physical vapor method. (c) FE-SEM image of the as-transferred horizo
electrodes on horizontal ZnONW arrays, which includes photolithography, m
Au electrodes. Inset: demonstration of an as-fabricated nanogenerator. The
Reproduced with the permission of ref. 12.
18954 | J. Mater. Chem., 2011, 21, 18946–18958
electric energy was effectively stored using capacitors, and it
was used successfully to light up a commercial LED, which is
a landmark progress toward building self-powered devices by
harvesting the energy from the environment. The authors used
an effective approach, named the scalable sweeping-printing-
method, to fabricate flexible nanogenerators with high effi-
ciency. This method consists of two main steps. In the first step,
the vertically aligned NWs were transferred to a receiving
substrate to form horizontally aligned arrays. The major
components of the transfer setup can be divided into two stages
(Fig. 11a). 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. The 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 trans-
ferred 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. In the second step, electrodes are
deposited to connect all the NWs together. Fig. 11 gives an
illustration of the fabrication process and structure character-
ization of the nanogenerator.
The working principle of the nanogenerator is illustrated by
the schematic diagrams in Fig. 12a and b. The NWs connected
in parallel contribute collectively to the current output; NWs
in different rows connected in series improve the voltage
anogenerator with a high output efficiency. (a) Experimental setup for
age of the as-grown vertically aligned ZnO NWs grown on a Si substrate
ntal ZnO NWs on a flexible substrate. (d) Process of fabricating the Au
etallization and lift-off. (e) FE-SEM image of ZnONW arrays bonded by
arrowhead indicates the effective working area of the nanogenerator.
This journal is ª The Royal Society of Chemistry 2011
Fig. 12 Working principle and output measurement of the nano-
generator. (a) Schematic diagram of the nanogenerator structure without
mechanical deformation, in which Au 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 nanogenerator. (d) Short circuit
current measurement of the nanogenerator. Reproduced with the
permission of ref. 12.
Fig. 13 Output of a nanogenerator as excited by the ultrasonic wave
when the UV light was turned on and off. Reproduced with the
permission of ref. 51.
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output constructively. The same growth direction of all NWs
and the sweeping printing method ensure that the crystallo-
graphic orientations of the horizontal NWs are aligned along
the sweeping direction. Consequently, the polarity of the
induced piezopotential is also aligned, leading to a macro-
scopic potential contributed constructively by all the NWs
(Fig. 12b). To examine the performance of the highly efficient
output nanogenerator, a linear motor was used to periodically
deform the nanogenerator in a cyclic stretching-releasing
agitation (0.33 Hz). The open-circuit voltage and short-circuit
current were measured with caution to exclude possible arti-
facts. At a strain of 0.1% and a strain rate of 5% s�1, the peak
voltage and current reached up to 2.03 V and 107 nA,
respectively.
Nanogenerator fabricated with lateral ZnO NWs array
produced higher voltage as compared to nanogenerator fabri-
cated with PZT nanofibers. It is due to the same growth direction
of ZnO NWs guarantee the alignment of the piezoelectric
potentials in all the NWs and successful scaling up of the output.
On the other hand, PZT nanofibers are not perfectly aligned and
scaling up of the output is not as strong as in ZnO NWs.
5. The important factors that affect the performanceof a nanogenerator
Carrier density
The carrier density of the semiconductor piezoelectric nano-
materials is one of the important factors that affect the
performance of a nanogenerator. Liu et al. found that a higher
carrier density increases the rate at which the piezoelectric
This journal is ª The Royal Society of Chemistry 2011
charges are screened/neutralized but a very low carrier density
prevents the flow of current through the NWs and increases the
inner resistance of the NWs.51–53 The carrier density needs to be
high enough to transport the current under the driving of the
piezoelectric potential in the charge releasing process. There-
fore, there should be an optimum conductance of the NW to
maximize the output of the NG. They demonstrated a series of
experiments to tune the carrier density with ultraviolet (UV)
light irradiation on the nanogenerator and observed it. Fig. 13
shows the output demonstration of a nanogenerator, as excited
by an ultrasonic wave when the UV light is turned on and off.
This confirms that a large number of carriers screen the
piezoelectric potential and reduce the output of the nano-
generator. UV light reduces the output current by 30–45%,
indicating that an increase in carrier density is not beneficial for
improving the output power.
Schottky contact
The mechanism of the nanogenerator is based on two
important physical quantities. One is the optimum conduc-
tivity and carrier density of the piezoelectric semiconductor
NW in the first step to prevent neutralization of the piezo-
electric potential distribution. The second is the height of the
Schottky barrier, which needs to be high enough to hold the
charges from leaking. Schottky contact between the metal
contact and piezoelectric semiconductor NW is a key factor to
the current generation process. Examining how it affects the
performance of nanogenerator will provide effective guidance
in designing and fabricating high output nanogenerators. Liu
et al. characterized the current–voltage (I–V) characteristics of
a set of nanogenerators to illustrate the effects of the Schottky
barrier for current generation.51 Fig. 14a shows the I–V
characteristics of a nanogenerator that did generate an output
current (top inset of Fig. 14a). The experimental setup for the
measurement is shown in the inset at the lower right corner in
Fig. 14a.
Electrodes conductivity
In addition to the fact that output power varies with the crys-
talline quality, length and contact area of the NWs,54 the
J. Mater. Chem., 2011, 21, 18946–18958 | 18955
Fig. 15 Output current densities of graphene-based nanogenerators
using graphene electrodes with various sheet resistances. Reproduced
with the permission of ref. 47.
Fig. 14 I–V characteristics of an assembled NG to identify its perfor-
mance for producing current. (a) For a nanogenerator that actively
generates current, as shown in the inset, its I–V curve when the ultrasonic
wave was off clearly shows Schottky diode behavior. UV light not only
increased the carrier density (or conductivity) but also might reduce the
barrier height. (b) For a ‘‘defective’’ nanogenerator that did not produce
a current, the I–V curve shows ohmic behavior. Reproduced with the
permission of ref. 51.
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electrical conductivity of the electrodes used to integrate the
nanogenerator also plays a major role in enhancing the power
generation from the nanogenerators. Our group reported that
the use of lower sheet resistance electrodes contributes to a higher
output current. The higher conductivity of the electrodes ensures
that a larger number of electrons contribute to the output current
in an external circuit. Shin et al. reported the modulation of
current generation by varying the sheet resistance of graphene,47
as shown in Fig. 15.
6. Further discussion
There are several successful demonstrations on nanogenerators
as summarized in Table 1. There is still scope for the further
improvement in the output performance through scientific find-
ings, such as neutralization of piezoelectric potential screening
effect due to the free carriers into semiconductor NWs, optimi-
zation and localization of free carriers in NWs, which affect any
piezoelectric signals. Alexe et al.55 discussed in their work that
a high intrinsic conductivity of the piezoelectric material itself is
detrimental to the detection of any piezoelectric signal. They
argued that charge generated by piezoelectric effect will be
cancelled due to high electron mobility (�100 cm2 V�1 s�1) and
high free carrier concentration (�1018 cm�3) in the ZnO NWs.
18956 | J. Mater. Chem., 2011, 21, 18946–18958
Therefore, the detection of piezoelectric signals becomes a diffi-
cult task.
Wang replied in many aspects of his correspondence
article56 and concluded that Alexe et al. used a measurement
system of possible artifacts such as huge noise level, inter-
ference from the environment and instantaneous system errors
during the measurement. Consequently, their measurement
system could not detect piezoelectric signals. He predicted and
verified with the experiment that conductivity and free carriers
can screen the piezoelectric charge but they cannot totally
cancel out the piezoelectric charges, which means that
magnitude of the piezoelectric potential will be reduced by the
free carriers.
Previous experimental results and findings support Wang’s
claim. ‘‘Switching-polarity’’ test in research works confirms and
verifies that the measured signals were from NWs based
nanogenerators rather than the measurement system. His
argument is ‘‘free carriers can screen the piezoelectric charge
but they cannot totally cancel out the piezoelectric charges’’. It
could be due to atomic density of ZnO of the order of about
1022 cm�3 which are much larger than intrinsic free carriers
density of about 1017 cm�3. Hence, although magnitude of the
piezoelectric potential is reduced by the free carriers, it is hard
This journal is ª The Royal Society of Chemistry 2011
Table 1 Summary of materials used in fabricating the nanogenerators and their measured output performancesa
Material Synthesis Bandgap/eVElectronaffinity/eV
Length (L),diameter (D)
Devicetype
Output performance
ReferenceVoltage CurrentCurrentdensity
Power orpower density
n-ZnO PVD 3.37 4.5 L z 50 mm AC 2.03 V 107 nA — 11 mW cm�3 12D z 200 nm
PZT Electro-spinningprocess
3.4 2.15 L z 500 mm AC 1.63 V — — 0.03 m W 28D z 60 nm
PZT Hydrothermalprocess
3.4 2.15 L z 5mm AC 0.7 V — 4mA cm�2 2.8 mW cm�3 8D z 500 nm
PVDF Electro-spinningprocess
9.23 �0.53 L z 6.5 mm AC 5–30 mV 0.5–3 nA — — 27D z 500 nm
CdS PVD 2.5 4.8 L z 1 mm — 3 mV — — — 21D z 100 nm
CdS Hydrothermal 2.5 4.8 L z 1 mm — 0.5–1 mV — — — 21D z 100 nm
BaTiO3 High temperaturechemicalreaction growth
3.3 3.90 L z 15 mm AC 25 mV — — — 7D z 280 nm
ZnO–ZnS Thermalevaporationand etching
3.3–3.6 4.5–3.9 — — 6 mV 23
GaN CVD 3.4 4.1 L z 10–20 mm DC 20 mV — — — 24,25D z 25–70 nm
InN VLS 0.7–0.9 5.8 L z 5 mm DC 1.0 V — — — 26D z 25–100 nm
n-ZnO Solution growth 3.37 4.35 L z 2mm DC — — 2 mA cm�2 — 46D z 100 nm
n-ZnO Solution growth 3.37 4.35 L z 1.5–2mm DC — — 3.7 mA cm�2 — 32D z 100 nm
n-ZnO CVD 3.37 4.35 L z 10 mm AC 50 mV — — — 3D z 150 nm
n-ZnO CVD 3.37 4.35 L z 3 mm DC 20 mV — 0.5 mA cm�2 — 36D z 90 nm
a PVD ¼ physical vapor deposition, CVD ¼ chemical vapor deposition and ‘‘—’’ ¼ not stated.
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to support that charge generated by piezoelectric effect in the
ZnO NW is fully cancelled out by the free carriers. These were
very useful arguments with the detailed discussion from Alexe
et al. and Wang to enhance the scientific and technical under-
standing of nanogenerators.
Concluding remarks
The importance of nanogenerators and the recent advances in
power generation through nanogenerators was reviewed along
with the power generation mechanism. Recent advances in
piezoelectric nanogenerators open many doors for power
generation through ambient energy harvesting for practical real
world applications. Piezoelectric nanogenerators are promising
for the miniaturization of a power package and the self-powering
of nano-systems used in implantable bio-sensing, environmental
monitoring and personal electronics. In particular, flexible and
foldable nanogenerators are useful in areas that require a fold-
able or flexible power source, such as implanted biosensors in the
muscle or joint, and have the potential of directly converting
biomechanical or hydraulic energy in the human body, such as
flow of body fluid, blood flow, heartbeat, and contraction of
blood vessels, muscle stretching or eye blinking, into electricity to
power implanted bio-devices. Sound-driven nanogenerator
converts sound energy from speech, music or noise into electrical
power for novel applications including mobile phones that can be
This journal is ª The Royal Society of Chemistry 2011
charged during conversations. Considerable research and engi-
neering efforts are being made to enhance power generation
through nanogenerators to commercialize its applications not
only in nano-systems, but also to power microelectronic devices.
There have been successful demonstrations that strong enough
electrical power generated through nanogenerators can contin-
uously drive a commercial LCD, light up a commercial LED and
LD, which confirm the feasibility of using nanogenerators for
powering mobile and personal microelectronics. More effort will
be needed to fabricate integrated amplifier with power generators
to amplify the output power to meet the world’s energy demand.
There should be efforts in harvesting multiplex energy through
the nanogenerators for efficient electricity generation. In addi-
tion, more study will be needed for DC power generation
through nanogenerators and enhancing their output for the
miniaturization of a power package.
Acknowledgements
This research was supported by the International Research &
Development Program of the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and
Technology (MEST) (2010-00297) and by Basic Science
Research Program through the NRF funded by the MEST
(2009-0077682 and 2010-0015035), and by the New &Renewable
Energy of the Korea Institute of Energy Technology Evaluation
J. Mater. Chem., 2011, 21, 18946–18958 | 18957
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and Planning (KETEP) grant funded by the Korea Government
Ministry of Knowledge Economy (No. 2009T100100614).
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