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Cite this: J. Mater. Chem., 2011, 21, 18946

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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:kimsw1@skku.edu; 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.

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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

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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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