Highly efficient flexible piezoelectric nanogenerator and femtosecond two-photonabsorption properties of nonlinear lithium niobate nanowiresManoj Kumar Gupta, Janardhanakurup Aneesh, Rajesh Yadav, K. V. Adarsh, and Sang-Woo Kim

Citation: Journal of Applied Physics 121, 175103 (2017); doi: 10.1063/1.4982668View online: http://dx.doi.org/10.1063/1.4982668View Table of Contents: http://aip.scitation.org/toc/jap/121/17Published by the American Institute of Physics

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Highly efficient flexible piezoelectric nanogenerator and femtosecondtwo-photon absorption properties of nonlinear lithium niobate nanowires

Manoj Kumar Gupta,1,a) Janardhanakurup Aneesh,1 Rajesh Yadav,1 K. V. Adarsh,1

and Sang-Woo Kim2

1Department of Physics, Indian Institute of Science Education and Research, Bhopal, Bhopal Bypass Road,Bhauri, Bhopal, Madhya Pradesh 462066, India2School of Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology(SAINT), Center for Human Interface Nanotechnology (HINT), and IBS Center for Integrated NanostructurePhysics, Institute for Basic Science (IBS), Sungkyunkwan University (SKKU), Suwon 440-746, South Korea

(Received 7 March 2017; accepted 15 April 2017; published online 3 May 2017)

We present a high performance flexible piezoelectric nanogenerator (NG) device based on the

hydrothermally grown lead-free piezoelectric lithium niobate (LiNbO3) nanowires (NWs) for scav-

enging mechanical energies. The non-linear optical coefficient and optical limiting properties of

LiNbO3 were analyzed using femtosecond laser pulse assisted two photon absorption techniques for

the first time. Further, a flexible hybrid type NG using a composite structure of the polydimethylsi-

loxane polymer and LiNbO3 NWs was fabricated, and their piezoelectric output signals were mea-

sured. A large output voltage of �4.0 V and a recordable large current density of about 1.5 lA cm�2

were obtained under the cyclic compressive force of 1 kgf. A subsequent UV-Vis analysis of the as-

prepared sample provides a remarkable increase in the optical band gap (UV absorption cut-off,

�251 nm) due to the nanoscale size effect. The high piezoelectric output voltage and current are dis-

cussed in terms of large band gap, significant nonlinear optical response, and electric dipole align-

ments under poling effects. Such high performance and unique optical properties of LiNbO3 show

its great potential towards various next generation smart electronic applications and self-powered

optoelectronic devices. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4982668]

INTRODUCTION

Lithium niobate (LiNbO3) is one of the most important

materials, which attracted massive attention because of its

fascinating physical properties such as piezoelectricity, ferro-

electricity, and nonlinear optical properties.1–3 LiNbO3 exhib-

its potential for novel applications by coupling the excellent

photostability, large piezoelectric charge coefficient, electro-

optic, nonlinear optical coefficients, and a large indirect band

gap of around 3.37 eV.4–7 Due to excellent electro-optic coef-

ficients, high thermal, mechanical, and chemical stabilities

and wide intrinsic bandwidth, LiNbO3 has wide scientific and

technological applications such as ultrafast modulation, fre-

quency conversion, tunable filter, and electro-optic modula-

tion.3,8–12 Further, several properties of LiNbO3 have been

extensively studied in the bulk form, but due to difficulties in

synthesizing at nanometer scales, its behaviour at nano-

dimensions is not well understood.13

Moreover, LiNbO3 is a well-known lead-free ABO3-type

perovskite that exhibits room-temperature ferroelectricity

with a larger spontaneous polarization of about 70 lC cm�2

and high ferroelectric transition temperature (�1483 K).5,14,15

The room temperature piezoelectricity and ferroelectricity of

LiNbO3 make it as a potential candidate for a new innovative

application as a flexible piezoelectric nanogenerator (NG) for

harvesting mechanical energy from the living environment

and the human body. For example, the mechanical energy

generated in the form of regular or irregular vibrations, such

as flow of body fluid, heartbeat, contraction of blood vessels,

muscle stretching eye blinking, walking, sound waves, air

flow, and so on, can be easily harvested by NGs.16–21 Various

NGs that convert mechanical energy into electrical energy

via the piezoelectric effect have been developed based on the

piezoelectric one-dimensional nanostructures of ZnO, InN,

GaN, KNbO3, and lead zirconate titanate (PZT) nanorods/

nanowires (NWs)/thin films and piezoelectric polymers.22–28

However, under mechanical pressure, the physical damage

of the device and breaking of NWs are most common

issues.28,29 Therefore, realization of high performance and

sustainable piezoelectric power output without damaging the

device is a challenging task for operating small electronic

devices such as light emitting diodes (LEDs), liquid crystal

displays (LCDs), and various self-powered nanosystems.

Moreover, it has been reported that as the size of the

particle reduces to the nanoscale, a drastic variation may

occur in their optical and electrical properties.30–33

Therefore, it is urgently required to understand the light

matter interactions in NWs of LiNbO3 and to measure its

non-linear optical coefficient for optoelectronic applications.

Among various techniques, the z-scan method is a popular,

simple, and effective tool for determining nonlinear proper-

ties, which provides the magnitudes of the real and imaginary

parts of the nonlinear susceptibility along with the sign of the

real part.34–36 Further, it has been shown that the pulse dura-

tion typically in the nano and pico-second time scales

strongly influences the nonlinearity.37,38 Therefore, determin-

ing nonlinearity in the femto-second time regime is a subject

a)Author to whom correspondence should be addressed. Electronic mail:

mkgupta@iiserb.ac.in

0021-8979/2017/121(17)/175103/7/$30.00 Published by AIP Publishing.121, 175103-1

JOURNAL OF APPLIED PHYSICS 121, 175103 (2017)

of great interest in order to design electro-optic and photonic

devices.39–42

In this work, we report the synthesis of LiNbO3 NWs

using the low-cost hydrothermal route for high efficient

mechanical to electric energy conversion and optoelectronic

applications. Structural and morphological characterizations

of LiNbO3 are also carried by X-ray diffraction (XRD), scan-

ning electron, and high-resolution transmission electron

microscopes (HR-TEM). Nonlinear optical properties were

investigated by the z-scan method using femtosecond laser

pulses. By utilizing the polydimethylsiloxane (PDMS) poly-

mer and LiNbO3 NWs, a composite-type flexible NG device

is developed. Through a periodic impact on the composite

NG by a linear motor, a piezoelectric output voltage and cur-

rent were obtained.

EXPERIMENTAL

Synthesis and fabrication of a LiNbO3 basednanogenerator

To synthesize the LiNbO3 NWs, high purity Nb2O5

and LiOH were mixed in the ratio of 1:10 in the distilled

water and subsequently stirred for 2 h. The mixture is

poured in a 50 ml Teflon lined stainless steel autoclave and

heated at 180 �C for 3 days in the oven. The white precipi-

tate was obtained after the mixture was cooled down to

room temperature. The final white product was washed

with distilled water and ethanol several times and dried at

120 �C for 12 h.

To fabricate flexible piezoelectric NGs, the powder of

LiNbO3 NWs was homogeneously mixed with the PDMS

polymer, and the resultant composite was spin-coated on an

indium tin oxide (ITO)-coated polyethylene terephthalate

(PET) substrate at 500 rpm for 15 s. A thin layer of the Cr/Al

film deposited on a plastic substrate serving as the electrode

of the piezoelectric NG was mechanically integrated on top

of the composite film. The effective area and thickness of the

device were 2 cm� 3 cm and 600 lm, respectively.

Characterization and measurements

The crystal structure of as-grown samples were investi-

gated by X-ray diffraction (XRD) with a Bruker D8 Advance

Cu-Ka1, k¼ 1.5401 A. Field-emission scanning electron

microscopy (FE-SEM) measurements were performed for

the morphological investigation. TEM images were obtained

using a JEOL JEM-2100F microscope. The UV-Vis optical

absorption spectrum was recorded using the Agilent Cary

5000 UV-VIS-NIR spectrophotometer. The third-order non-

linear optical properties of the LiNbO3 NWs were deter-

mined by the z-scan method using a femtosecond laser

system. A pushing tester (Labworks Inc., model no. pa-151

and ET-126B-4) was used to apply mechanical force to the

device. The output electric signal of the NG was recorded by

using a Tektronix DPO 3052 Digital phosphor oscilloscope

and a low-noise current preamplifier (model no. SR570,

Stanford Research Systems).

RESULTS AND DISCUSSION

The phase identification and the confirmation of crystal-

linity of the as-grown LiNbO3 NWs were investigated by the

powder XRD technique. The obtained XRD pattern of

LiNbO3 NWs at room temperature is shown in Fig. S1 (sup-

plementary material). The pattern shows that the diffraction

peaks are in agreement with JC-PDS #85-2456 and the for-

mation of the rhombohedral crystal system of the space

group R3c (166) with lattice constants a¼ b¼ 5.148 A and

c¼ 13.858 A.43 In addition to the LiNbO3 phase, small peaks

corresponding to the secondary phase of lithium niobate

(LiNb3O8) are also detected.

A typical FE-SEM image of the LiNbO3 NWs is shown

in Fig. 1(a), which reveals a well-defined NW morphology

with a diameter in the range of 250–500 nm and a length of

about 10–15 lm. HR-TEM was employed to study the size

and crystalline quality of the LiNbO3 NW (Figs. 1(b)–1(d)).

The HR-TEM image exhibits clear fringes with spaces of 0.

375 nm and 0.170 nm, which are in good agreement with the

interplanar spacing of (012) and (116) planes (Fig. 1(c)). The

crystalline nature of the LiNbO3 NW can be confirmed by its

corresponding fast Fourier transform (FFT) pattern. The dif-

fraction spots can be indexed to the rhombic phase of

LiNbO3 where regular and bright spots of the selected area

electron diffraction (SAED) indicate the crystalline feature

(Fig. 1(d)).

Figure 2 depicts a UV–Vis optical absorption spectrum

of the as-prepared LiNbO3 NWs, which shows a strong

absorption band-edge around 250 nm that is significantly

smaller than the bulk LiNbO3 single crystal (315 nm) by Guo

et al.44 Interestingly, the observed band gap of lithium nio-

bate in the present study is well correlated with a theoretically

calculated band gap value (�4.71 eV) for the ferroelectric

phase of LiNbO3.45 The obtained cut-off indicates that as-

grown LiNbO3 NWs have comparatively larger band gap

than previous reports, which facilitates its versatile applica-

tions in optoelectronic devices.44–47 The high value of the

band gap is attributed to the size-quantization effect induced

discrete energy spectrum in the conduction band and forma-

tion of energy levels in the forbidden gap appeared due to

surface defects.45 It is also expected that the exciton energy

spectrum modified by spatial confinement of the exciton

wave functions can result in an increase in the band gap of

LiNbO3.45,47 Further, it is well known that piezopotential

generation is significantly affected by free charge carriers of

LiNbO3 NWs. Therefore, the optical band gap of LiNbO3

also plays an important role in defining piezoelectric output

voltage/current because of the piezoelectric potential screen-

ing effect induced by the presence of the free carrier density.

Therefore, a comparatively large band gap of as grown

LiNbO3 NWs may lead to a high output piezoelectric signal

under the mechanical pressure.

An open aperture z-scan technique was used for the mea-

surement of the nonlinear absorption (NLA) coefficient of

LiNbO3 NWs.48 The experiments were carried out at 560 nm

using 120 fs pulses generated by using an optical parametric

amplifier. The details of the experimental procedures are

described in the supplementary material. The obtained spectra

175103-2 Gupta et al. J. Appl. Phys. 121, 175103 (2017)

from open aperture z-scan measurement at two different

wavelengths are shown in Fig. 3(a). The measurements were

carried out with an on-axis peak intensity of 78 GW cm�2. It

can be seen that for 800 nm photon light, LiNbO3 NWs do not

show any significant non-linear absorption, whereas for

560 nm, it exhibits non-linear absorption. Further, a decrease

in transmittance as a function of input intensity is also

observed, which clearly indicates the presence of non-linear

absorption in the LiNbO3 NWs for 560 nm photons (Fig.

3(b)). This behavior of NLA in LiNbO3 NWs under two dif-

ferent excitation wavelengths can be easily understood in

terms of the band gap of LiNbO3 as shown in the schematic

diagram of Fig. S2. It is obvious that in the case of the 800 nm

laser beam, the photon energy (1.55 eV) is below the band

gap of LiNbO3. Therefore, transition of electrons from the

valence band to the conduction band is not possible for

800 nm photons, which results in no nonlinear absorption.

Interestingly, although the photon energy of 560 nm (2.21 eV)

is still less than the band gap of LiNbO3, the two photon

energy was sufficient to induce electron transitions from

the valence band to the conduction band in LiNbO3 NWs

as confirmed in the obtained result. Moreover, at the lower

intensity side, there is no significant absorption for 560 nm.

Nevertheless, at the higher intensity side, a significant reduc-

tion in transmittance is measured, which is due to the increase

in the density of photons that increases the probability of

simultaneous absorption of two photons (usually known as a

two photon absorption (TPA) process).

In the case of two photon absorption (TPA), the normal-

ized light transmittance for a laser pulse having both tempo-

ral and spatial Gaussian profiles is given by48

T ¼ 1

Q zð Þffiffiffippð1�1

ln 1þ Q zð Þx2� �

dx; (1)

where

Q zð Þ ¼bI0Lef f

1þ z

zr

� �2:

b is the two photon absorption coefficient, I0 is the peak

intensity at focus in the absence of the sample, Zr is the

Rayleigh length for the focusing lens, z is the sample posi-

tion with respect to the focus (z¼ 0), and Lef f ¼ 1�e�aLð Þa is

the effective length for a sample having thickness L, where ais the linear absorption coefficient.

The nonlinear absorption coefficient b was obtained by

curve fitting of Equation (1) for the experimental z-scan dataFIG. 2. UV-Vis spectrum of the LiNbO3 NWs.

FIG. 1. Scanning electron microscopy

(SEM) image (a) of the LiNbO3 NWs

with an average diameter of 300 nm

and a length in the range of 10–15 lm.

(b) Transmission electron microscopy

image. (c) The lattice fringe image

for (012) and (116) with its (d) FFT

transformation.

175103-3 Gupta et al. J. Appl. Phys. 121, 175103 (2017)

of the excitation wavelength at 560 nm. The two photon

absorption coefficient b is found to be 0.13 6 0.02 mm

GW�1. This value is slightly smaller as compared to the pre-

viously reported bulk form of LiNbO3.49 The decrease in the

b is due to the nanocrystalline size of LiNbO3, which is con-

sistent with the recent studies.50 However, in order to estab-

lish the relationship of the optical nonlinearity with crystallite

size, extensive work on varying sizes is required. The optical

limiting property of the LiNbO3 NWs was also investigated.

Fig. 3(c) shows the output intensity through the sample as a

function of input intensity. It can be seen that at the lower

intensity side, the output intensity is linearly proportional to

the input intensity; however, as the input intensity increases

up to a certain value, it starts deviating from its linear behav-

ior. The results indicate that LiNbO3 NWs are well suitable

for optical limiting applications such as protection of human

eyes and sensors from intense laser radiations, in which

blocking or limiting the high-intensity beams is required.

Further, owing to its well-known piezoelectric property

of LiNbO3, we demonstrated alternative-current (AC) power

generation from piezoelectric NGs as a potential application

of these LiNbO3 NWs. A robust flexible NG device was

designed to harvest mechanical energy from the living

environment. A composite type flexible NG was fabricated

using a device structure of the ITO coated PET substrate/

(LiNbO3:PDMS) with an aluminum (Al) top electrode as

shown in Fig. 4. An ITO coated PET substrate was used as

the bottom electrode and as a flexible substrate. The sche-

matic diagram of device fabrication is shown in Fig. S3.

Initially, a mixture of the LiNbO3 NW and PDMS polymer

with a volume ratio of 40:60 was prepared and then spin

coated on the ITO/PET substrate at 1000 rpm for 15 s. In

order to prepare the top electrode, a thin layer of Al was

deposited on the Cr coated flexible polyethylene-naphthalate

(PEN) substrate and then carefully mechanically integrated

with the top of the composite layer to form a complete NG

device. A schematic diagram and original photo image of the

practical device are shown in Figs. 4(a) and 4(b), respec-

tively. The FE-SEM analysis of the LiNbO3:PDMS sample

was carried out, to gain insights into the NW distribution

behavior, as shown in Figs. 4(c) and 4(d). The low and high

magnified SEM images confirmed that LiNbO3 NWs are

FIG. 3. Open aperture z-scan curve for LiNbO3 NWs dispersed in ethanol: (a) the variation of normalized transmitted intensity as a function of position, (b)

input intensity (solid line represents the theoretical fit), and (c) optical limiting curve showing the deviation from linear behaviour for higher input intensity.

FIG. 4. (a) Schematic diagrams of the

flexible PDMS assisted LiNbO3NW

based piezoelectric NG, (b) original

photo image of the flexible NG on the

transparent ITO/PET substrate and the

top view of the composite (inset), and

(c) low and (d) high magnified-SEM

images of the LiNbO3: PDMS composite.

175103-4 Gupta et al. J. Appl. Phys. 121, 175103 (2017)

randomly distributed inside the PDMS polymer. From the

cross-sectional FE-SEM images shown in Fig. S4, the thick-

ness of the composite layer was estimated to be about 350 lm.

Fourier transform infrared spectroscopy (FT-IR) spectra were

also measured in order to obtain physical insights into the

chemical bonding and phase formation in the PDMS:LiNbO3

composite (Fig. S5). The result of FT-IR is discussed in the

supplementary material.

The piezoelectric output voltage and current generated

from a composite LiNbO3:PDMS based hybrid NG were

obtained under periodic vertical pushing and releasing condi-

tions using the computer interface force simulator. An output

voltage of about 4.0 V and a current density of about 1.5 lA

cm�2 were obtained under a vertical compression of 1 kgf

at a frequency of 4 Hz (sinusoidal vibration) as shown in

Figs. 5(a)–5(d) and 6, respectively. The device was electri-

cally poled before the piezoelectric signal measurement with

a dc electric field of about 90 kV/cm at room temperature.

It is noteworthy that the electric dipoles can be easily ori-

ented or switched in a particular direction after applying a

suitable electric field due to the ferroelectric property of the

LiNbO3 NW.6,7

To confirm that the measured signal was from the

NG rather than the instruments, we performed “switching-

polarity” tests. As predicated, when the voltage meter/

current meter was forward connected to the NG (i.e., positive

and negative probes were connected to the positive and nega-

tive electrodes, respectively), positive pulses were recorded

during the pushing and when the voltage meter/current meter

was connected in reverse, the output pulses were also

reversed as shown for output voltage (Figs. 5(a) and 5(c))

and output current (Figs. 6(a) and 6(b)). Enlarged views of

one cycle of output voltage (Figures 5(b) and 5(d)) and out-

put current (Figs. S6 and S7 of supplementary material) were

given for further verification of the piezoelectric reversible

signal under pushing and releasing conditions. It is notewor-

thy that generated output voltage from the device was almost

10 times and output current was almost 150 times higher

than the previously reported LiNbO3 based NG (0.46 V,

9.11 nA).51 This drastic enhancement of output voltage and

FIG. 5. (a) Piezoelectric output voltage

generated from the LiNbO3 NW:PDMS

based NG under compressive force

(forward connection), (b) enlarged view

of one cycle of output voltage under

forward connection, (c) output voltage

under reverse connection (switching-

polarity test), and (d) enlarged view of

one cycle of output voltage under

reverse connection.

FIG. 6. (a) Piezoelectric output voltage

generated from the LiNbO3 NW:PDMS

based NG under compressive force (for-

ward connection) and (b) output current

under reverse connection (switching-

polarity test).

175103-5 Gupta et al. J. Appl. Phys. 121, 175103 (2017)

current may be due to the comparatively large band gap. It is

expected that due to the large band gap and therefore com-

paratively small free charge carriers in LiNbO3 NWs, the

piezoelectric screening effect reduces, which results in high

piezoelectric output performance. A comparison of output

performance of other reported composite type piezoelectric

NGs is given in the supplementary material (Table S1).

Further, it was also reported that the formation of the second-

ary phase such as LiNb3O8 (niobium-rich grain boundaries)

in the lithium niobate phase decreases the piezoelectric

charge coefficient, which indicates that the piezoelectric out-

put voltage/current can be further reduced.52 Interestingly, in

contrast to expectation, piezoelectric output voltage/current

from the as-grown LiNbO3 sample based NG (which con-

tains a small amount of the LiNb3O8 phase) was much higher

than the previously reported LiNbO3 based NG. Therefore,

we proposed that enhancement of piezoelectric output volt-

age and current from the NG device is mainly due to the dra-

matic increase in the band gap and reduction of screening of

piezoelectric polarisation charges by free carriers. It is worth

to point out that no significant output voltage was observed

from only the PDMS based NG under the same compressive

force.

The working principle of the piezoelectric NG device for

generating the electrical signal is presented in Fig. 7. In the

absence of any external pressure (and/or electric field), no

piezo-induced electric charge is generated due to zero net

dipole moment, and therefore, no electric signal is detected

(Fig. 7(a)). However, when an external pushing force is verti-

cally applied to the top electrode of the electric poled NG

device, the electric dipoles of LiNbO3 presented in the matrix

are strongly aligned due to the poling effect. Consequently, a

piezoelectric potential is created due to mechanical deforma-

tion of the device. As a result, overall polarization across the

electrodes changes and a significant piezoelectric potential

across the external electrodes is produced during pushing

(positive pulse). Further, when the vertical compressive force

is released, the piezoelectric effect is disappeared, and the

accumulated charges at the bottom electrode side move back

to the opposite direction and an electric signal in the reverse

direction is obtained (Fig. 7(b)).

CONCLUSIONS

In summary, we demonstrated the growth of large scale

synthesis of LiNbO3 NWs using the low-cost hydrothermal

solution technique. Nonlinear optical analysis and optical

limiting properties of LiNbO3 were investigated via the two

photon absorption technique using femtosecond laser pulses.

The flexible piezoelectric NG device was fabricated using

the composite of the PDMS polymer and lead-free LiNbO3

NWs. Under vertical periodic compressive force, a large out-

put voltage about 4.0 V and a very large current density of

about 1.5 lA cm�2 were obtained under the cyclic compres-

sive force. The output current was 150 times higher than the

previously reported LiNbO3 based NG. To align the electric

dipoles of ferroelectric/piezoelectric LiNbO3 inside the poly-

mer matrix, the electric poling method was employed. To

understand the enhancement of output performance, UV-ViS

and femtosecond laser studies were also carried out. We

have shown that large piezoelectric output voltage/current

from the composite NG is strongly affected by the large

band gap and nonlinear properties of LiNbO3. Such a result

can be exploited for harvesting mechanical energy and for

piezoelectronics and piezo-phototronic applications due to

their multifunctional properties.

SUPPLEMENTARY MATERIAL

See supplementary material for the XRD pattern of

LiNbO3 NWs, experimental details of two-photon absorp-

tion, the schematic image of two photon induced transition,

schematic diagrams of the piezoelectric nanogenerator

device fabrication process, the cross-sectional FE-SEM

image of the LiNbO3:PDMS composite, FT-IR spectra of the

LiNbO3:PDMS composite, the enlarged view of one cycle of

output current under forward and reverse connections under

mechanical stress, and a table showing the comparison of

output performance of the NG present here with other

reported composite type NGs.

ACKNOWLEDGMENTS

M.K.G. is grateful to the Department of Science &

Technology, Government of India, for awarding the DST-

INSPIRE Faculty Fellowship [IFA-13 PH-81].

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