Piezoelectric Nanotube Array for Broadband High Frequency ...

TUFFC-08319-2017 (Refer to published version: IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS,

AND FREQUENCY CONTROL, VOL. 65, NO. 3, 457-464, MARCH 2018)

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Abstract—Piezoelectric materials are vital in determining

ultrasonic transducer and imaging performance as they offer the

function for conversion between mechanical and electrical

energy. Ultrasonic transducers with high frequency operation

suffer from performance degradation and fabrication difficulty

of the demanded piezoelectric materials. Hence, we propose

one-dimensional (1D) polymeric piezoelectric nanostructures

with controlled nanoscale features to overcome the technical

limitations of high frequency ultrasonic transducers. For the

first time, we demonstrate the integration of well-aligned

piezoelectric nanotube array to produce high frequency

ultrasonic transducer with outstanding performance. We find

that nanoconfinement induced polarization orientation and

unique nanotube structure lead to significantly improved

piezoelectric and ultrasonic transducing performance over the

conventional piezoelectric thin film. A large bandwidth 126 %

(-6 db) is achieved at high center frequency, 108 MHz.

Transmission sensitivity of nanotube array is found 46 % higher

than monolithic thin film transducer attributed to the improved

electromechanical coupling effectiveness and impedance

match. We further demonstrate high-resolution scanning

ultrasonic imaging and photoacoustic imaging using the

obtained nanotube array transducers, which is valuable for

biomedical imaging application in the future.

Index Terms—nanotubes, piezoelectric, transducers, ultrasonic

I. INTRODUCTION

LTRASONIC transducers operating at frequencies over

tens or hundreds of megahertz (MHz) are essential to

achieve micrometer-scale resolution [1] for diagnostic

biomedical ultrasonic and photoacoustic imaging, and non-

destructive testing. The development of high frequency

ultrasonic transducer has been focusing on various thin film

materials [2], [3] as the required thickness of piezoelectric

elements shrinks with increasing operating frequency.

However, the performance of monolithic piezoelectric thin film

is known to degrade with the decreasing thickness [4]. 1-3

piezoelectric composite structure shows potential to improve

the performance over monolithic film, but the technical

challenge in producing nanoscale structural feature through top-

This project is partially supported by Institute of Materials Research and

Engineering (IMRE) A*STAR (Agency for Science, Technology and

Research), and by Singapore Maritime Institute under the Asset Integrity &

Risk Management (AIM) R&D Programme, Project ID: SMI-2015-OF-01, and

IMRE/15-9P1115.

W. H. Liew, K. Yao and S. Chen are with Institute of Materials Research

and Engineering (IMRE), A*STAR (Agency for Science, Technology and

down fabrication method limits the operating frequency of 1-3

composite transducer [5]. The trade-off between operating

frequency and performance has become the bottleneck for

advancing piezoelectric material and the high frequency

ultrasonic transducer technology.

Recent advances in one dimensional (1D) nanostructures

offer an opportunity for tuning material properties [6-9] through

nanoscale design and utilization of size effects. For instance,

the emerging piezotronics [10] harnesses the unique geometry

of various 1D piezoelectric nanostructures to create innovative

devices such as nanogenerators, [11], [12] piezoelectric diodes,

[13] and nano-force sensors [14]. However, the potential of 1D

piezoelectric nanostructures remains untapped for high

frequency ultrasonic transducing; a technology relies critically

upon low dimensional piezoelectric materials. The

electromechanical device applications of the 1D piezoelectric

nanostructures at high frequency over 100 MHz has not been

explored. We reckon that 1D piezoelectric nanostructure

derived from bottom-up approach with improved yield and

quality can be a potential technical solution to create ideal high

performance high frequency ultrasonic transducers.

Here we realize high frequency ultrasonic transducers with

center frequency up to 108 MHz using piezoelectric nanotube

array derived from bottom-up approach. We obtain nanotube

array comprising millions of nanotubes with well-aligned

structure and polarization, and uniform piezoelectric response

over a macro-scale area. We show that the nanotube array

structure with unique nanoscale effects is crucial to achieving

high performance ultrasonic transducers with enhanced

sensitivity and bandwidth. We further demonstrate high

resolution ultrasonic and photoacoustic imaging using the

obtained nanotube array ultrasonic transducers, illustrating the

practical application values of the high performance ultrasonic

transducers enabled with the nanotechnology strategy.

II. FABRICATION AND STRUCTURES

We fabricated high frequency ultrasonic transducers using

piezoelectric poly(vinylidene fluoride-co-trifluoroethylene)

(P(VDF-TrFE)) nanotube array as the active element (Fig. 1).

The P(VDF-TrFE) nanotubes were fabricated with template of

anodized alumina membrane [15] (AAM) as improved from our

Research), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634

([email protected]).

W. H. Liew and F. E. H. Tay are with NUS Graduate School for Integrative

Sciences & Engineering (NGS), National University of Singapore, 28 Medical

Drive, Singapore 117456

Piezoelectric Nanotube Array for Broadband

High Frequency Ultrasonic Transducer

Weng Heng Liew, Kui Yao, Senior Member, IEEE , Shuting Chen and Francis Eng Hock Tay

U

mailto:[email protected]



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previous process [16], [17]. Details of the fabrication in this

work are provided in the Appendix B. Fig. 1 a, b and c show the

nanotubes embedded in alumina matrix which separate the soft

nanotubes to form vertically aligned array without bundling.

The nanotubes form bundle if the alumina matrix is completely

removed (Fig. 1d). P(VDF-TrFE) nanotube array with different

nanotube’s dimensions (length: 1, 2, and 4 µm; outer diameter:

70 and 350 nm) were fabricated by controlling the AAM’s

processing parameters. Non-focused and focused high

frequency ultrasonic transducers were produced using flat and

curved nanotube array respectively. The curved nanotube array

was obtained on a curved anodized alumina membrane with the

focal length equal to the radius of the curvature. Piezoelectric

P(VDF-TrFE) thin film ultrasonic transducer was also

fabricated for performance comparison. Pulse-echo and

transmission characterizations were performed to evaluate the

performance of ultrasonic transducers including bandwidth and

sensitivity. The imaging performance of nanotube array

ultrasonic transducers were demonstrated with scanning

ultrasonic imaging and photoacoustic imaging systems.

III. ELECTROMECHANICAL PROPERTIES

In general, the performance of ultrasonic transducer depends

on the electromechanical coupling coefficient and acoustic

impedance of the piezoelectric elements. The square of

electromechanical coupling coefficient, k2, describes the energy

conversion effectiveness between electrical and mechanical

energy of piezoelectric materials. Higher electromechanical

coupling coefficient is desired for broader bandwidth with

higher imaging resolution and sensitivity. It is well known that

the maximum electromechanical coupling effectiveness occurs

at k33 mode of a high aspect ratio piezoelectric element, [18]

which refers to electromechanical conversion along the

electrical poling direction of the piezoelectric element.

However, the k33 mode resonance at frequency over 100 MHz

along thickness direction requires a small thickness in

micrometer-scale, such as a 100 MHz P(VDF-TrFE) ultrasonic

transducer corresponds film thickness of about 12 µm. The

lateral dimension of such piezoelectric elements needs to be in

sub-micrometer-scale to retain a high aspect ratio and hence

minimize in-plane clamping for achieving superior k33 mode

energy conversion. Such requirement is desirably fulfilled by

nanotube array which consists of massive number of aligned

nanotubes with high aspect ratio and minimum in-plane

mechanical constraint.

Electromechanical coupling coefficient of the P(VDF-TrFE)

nanotube array is related to material properties by the following

equation,

𝑘332 = 𝑑33

2 (𝑠33𝐸 𝜀33

𝑇⁄ ) (1)

where d33 is the effective piezoelectric strain coefficient, 𝑠33𝐸 is

the effective elastic compliance at constant electric field and 𝜀33𝑇

is effective dielectric constant at constant stress. These

properties of P(VDF-TrFE) nanotube array are favorably

altered by nanoconfinement effect during crystallization and

geometrical shape of resulting nanotubes. The

nanoconfinement effect imposed by the geometrical constraint

[17] in alumina nano-pores during the polar P(VDF-TrFE)

crystallization promotes the preferred orientation of polymer

chains and polarization in nanotubes. The dipoles in P(VDF-

TrFE), which are perpendicular to the polymer chains, are

aligned along the long axes of the nanotubes (Fig. 1d inset). The

alignment of the polarization along the axis significantly

enhances the effective d33 of the P(VDF-TrFE) nanotubes.

Apart from the desired polarization orientation, in-plane

Fig. 1. Structures of 1D piezoelectric nanotube array and ultrasonic transducer. Scanning electron microscope (SEM) images of piezoelectric nanotube array

showing (a) nanotubes embedded in alumina matrix and (b) uniformity over a large area. (c) Schematic illustration of piezoelectric nanotube array

embedded in alumina matrix with electrodes deposited at two ends of nanotube array. (d) SEM image of the piezoelectric nanotubes after removal of

alumina matrix, inset illustrating the polarization orientation of P(VDF-TrFE) along the nanotube axis. (e) (left) Design of the ultrasonic transducer with the

piezoelectric nanotube array and (right) picture of the device before Au electrode deposition, exposing the transparent nanotube array.



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mechanical clamping effect is negligible for nanotubes with the

thin nanotube wall (~ 70 nm) and the high aspect ratio. It is well

known that the in-plane clamping, such as from substrate

constraint, reduces the effective piezoelectric coefficient by

restricting the lateral movements in the piezoelectric materials

[19].

Both the nano-confinement induced polarization orientation

along the nanotube axis and the minimized in-plane clamping

as described above significantly enhance the effective

piezoelectric coefficient d33 of P(VDF-TrFE) nanotubes by 57

% to -28.0 pC N-1, from -17.8 pC N-1 of the thin film counterpart

(Table I and Appendix, Fig. A1).

TABLE I. Comparison of piezoelectric P(VDF-TrFE) thin film and nanotube

array.

Thin Film

[20], [21] Nanotube Array

𝑑33(𝑒𝑓𝑓) (pC N-1) -17.8 -28.0

𝑠33(𝑒𝑓𝑓)𝐸 (x10-10 m2 N-1) 2.15 4.45

𝜀33(𝑒𝑓𝑓)𝑇 (ε0) 13.2 7.7

𝜌(𝑒𝑓𝑓) (kg m-3) 1879 1202

Calculated 𝑔33(𝑒𝑓𝑓) (mm V N-1) -152.3 -410.7

Calculated 𝑘332 0.0121 0.0256

Calculated Z (MRayl) 2.95 1.63

Calculated T (% pressure) 67 95

While the effective dielectric constant 𝜀33𝑇 of P(VDF-TrFE)

nanotube array was significantly lower than thin film due to the

voids in the hollow nanotubes, the hollow structures lead to

higher effective elastic compliance 𝑠33𝐸 of nanotubes as shown

by the finite element modeling (see Appendix, Fig. A3). The

𝑠33𝐸 of nanotube array is not affected by the alumina matrix since

the nanotubes are mechanically detached from the partially

etched anodized alumina, as observed in Fig. 1a. After taking

into account the effects of 𝑑33, 𝜀33𝑇 , 𝑠33

𝐸 , the 𝑘332 of nanotube

array is 110 % higher than monolithic thin film, improved from

0.0121 to 0.0256, as shown in Table I.

The ultrasonic sensitivity of ultrasonic transducer in term of

voltage output is largely determined by piezoelectric voltage

coefficient (𝑔33), which describes the induced voltage under

applied pressure wave. The 𝑔33 is related with 𝑑33 and 𝜀33𝑇 as

in

𝑔33

= 𝑑33 𝜀33𝑇⁄ (2)

The g33 is dramatically improved from -152.3 mmV N-1 of thin

film to -410.7 mmV N-1 of nanotube array, due to the much

smaller 𝜀33𝑇 and larger 𝑑33of P(VDF-TrFE) nanotube array.

Thus, the significantly improved 𝑘332 and 𝑔33of nanotube array

are promising for producing broadband and sensitive high

frequency ultrasonic transducers.

IV. ACOUSTIC IMPEDANCE

Another key parameter for ultrasonic transducing

performance is the acoustic impedance (Z) of the piezoelectric

element. High acoustic impedance mismatch between the

piezoelectric element and the surrounding medium (which is

mostly water based such as bio-tissues, Z ≈ 1.5 MRayl) causes

low transmission of ultrasonic wave into the active element.

Acoustic matching layer is usually required in the conventional

ultrasonic transducer to facilitate the ultrasonic transmission.

However, such method is very difficult to apply on high

frequency ultrasonic transducer due to the stringent

requirements on the thickness and acoustic impedance of

matching layer. Nanotube array has significantly lower acoustic

impedance than thin film, as shown in Table I, and hence is able

to eliminate the necessity of acoustic matching layer. The low

acoustic impedance of nanotube array,

𝑍𝑝 = √𝐸𝜌 (3)

is determined by the low volumetric mass density (ρ) and the

low elastic modulus (E) of the hollow tube structure. We

examined the ultrasonic wave transmission by time domain

simulation using the k-space pseudospectral method, [22] and

found the ultrasonic transmission is as high as 95% for

nanotube array in water due to the near perfect acoustic

impedance matching with water. By contrast, the ultrasonic

transmission for P(VDF-TrFE) thin film is only 67 %. Hence,

piezoelectric nanotube array can achieve high ultrasonic

transmission in water based mediums including living tissues

without any extra acoustic matching layers.

V. ULTRASOUND CHARACTERIZATION AND IMAGING

The performance of piezoelectric nanotube array for

ultrasonic transducing was characterized through both pulse-

echo and transmission measurements. Pulse-echo

measurements involve both ultrasonic wave generating and

sensing by the same transducer to investigate the bandwidth and

pulse-echo sensitivity. A 30 MHz-ultrasonic transducer

(Olympus IMS) was used as the ultrasonic pulser in the

transmission measurement to investigate the ultrasound sensing

capability of piezoelectric nanotube array. Time domain signals

and Fourier spectra (after Fast Fourier Transform, FFT) for

pulse-echo and transmission measurements are shown in Fig. 2.

Fig. 2a shows that the 4 µm-nanotube array covers broader

frequency band (42 MHz) than thin film (34 MHz) with similar

center frequencies. By reducing the length of nanotubes, the

nanotube array ultrasonic transducer shows significant

broadening of bandwidth at much higher frequencies (120 % (-

6 dB) at 65 MHz and 126 % (-6 dB) at 108 MHz). The

longitudinal resonance frequency of the nanotubes increases

with the shortening of nanotubes and determines the center

frequency of the nanotube array ultrasonic transducer.

The broadband performance at high frequency indicates that

the nanotube arrays retain high piezoelectric performance

although the outer diameter and length of the nanotubes shrink

to 70 nm and 1 µm, respectively. It is also worth noting that the

120 MHz (nominal) PVDF film transducer shows similar



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bandwidth with the P(VDF-TrFE) thin film transducer and both

are outperformed by nanotube array transducers. The

broadband high frequency nanotube array ultrasonic transducer

can achieve axial resolution as high as 22 µm with the 1 µm

nanotubes as shown in Fig. 2b. The decreasing trend of

sensitivity with center frequency of ultrasonic transducer is due

to the higher attenuation of ultrasonic waves at higher

frequency while nanotube array shows 31 % higher sensitivity

than thin film with similar center frequencies.

In transmission mode measurement, the 4 µm-nanotube array

outperforms thin film by 46 % higher in ultrasonic sensitivity

as shown in Fig. 2d. Although both show comparable fractional

bandwidth (108 % (-6dB)) in transmission mode (Fig. 2c), the

nanotube array demonstrates better response at higher

frequency and covers a wider frequency band (18 MHz – 54

MHz) as compared to thin film (13 MHz – 44 MHz). The

enhanced electromechanical performance properties and

acoustic impedance matching for the nanotube array contribute

to the superior ultrasound sensing performance for nanotube

array. Ultrasonic sensitivity of nanotube array can potentially

be further improved by optimizing the transducer designs as the

piezoelectric voltage constant (g33) of nanotube array is

calculated to be ~2.7 times of that for thin film (Table I). The

outstanding sensing performance at high frequency is very

valuable for high resolution ultrasound imaging as the

resolution of ultrasonic imaging system increases with the

operating frequency and bandwidth of ultrasonic transducer.

Such high resolution and sensitivity can be utilized in

biomedical imaging for microscopic blood vessels such as

arteriole, venule and capillary. The practical application of piezoelectric nanotube array for

high frequency ultrasonic imaging was demonstrated through

the scanning ultrasonic imaging system as shown in Fig. 3a.

Focused ultrasonic transducers with focal length of ~2.6 mm

(see Appendix, Fig. A5) and center frequency of 40 MHz and

108 MHz were fabricated with P(VDF-TrFE) nanotube array

on curved anodic alumina membrane. Fig. 3b shows the high

resolution ultrasonic image of copper micro wires with

resolution of 20 µm obtained by the 108 MHz focused nanotube

array transducer, as compared to the image with resolution of

50 µm from 120 MHz (nominal) focused PVDF film ultrasonic

transducer (Fig. 3c). The results in Fig. A6a and A6c in the

Appendix show that the imaging resolution from nanotube

array ultrasonic transducers improves from 41 µm to 20 µm as

the center frequencies increases from 40 MHz to 108 MHz.

Photoacoustic imaging was demonstrated using the non-

focused 40 MHz nanotube array ultrasonic transducer (Fig. 4a).

Four strands of hair were used as the imaging targets and the

non-focused ultrasonic transducer mechanically scans through

the hairs to obtain a cross section image of the hairs. A laser

source with pulse duration of ~10 ns and wavelength of 700 nm

was used to excite the ultrasound waves on the imaging targets.

The hairs with diameters of ~ 80 µm and offset in vertical

positions can be observed using the nanotube array transducer

(Fig. 4b).

Fig. 2. Pulse-echo testing for P(VDF-TrFE) thin film, commercial PVDF film, 1µm, 2 µm and 4 µm- P(VDF-TrFE) nanotube array transducers. (a) Fourier

spectra shows the broadening of bandwidth with nanotubes and comparison with film-based transducers. (b) Sensitivity after compensated for lower pulsing

pressure (Appendix, Fig. A4) and the calculated axial resolution for thin film and nanotube array ultrasonic transducers. Transmission testing with P(VDF-

TrFE) thin film and 4 µm- P(VDF-TrFE) nanotube arrays as sensor and the commercial PVDF ultrasonic transducer as pulser. (c) Fourier spectra shows the

broader bandwidth and (d) time domain signal shows the higher sensitivity of nanotube array as ultrasound sensor.



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Fig. 3. (a) Schematic diagram of the scanning ultrasonic configuration with

focused ultrasonic transducer using curved nanotube array. Ultrasonic images

produced by (b) 108 MHz focused nanotube array transducer and (c) 120 MHz

(nominal) focused PVDF film ultrasonic transducer.

Fig. 4. (a) Schematic diagram of the photoacoustic scanning configuration with

ultrasonic transducer using flat nanotube array. (b) Photoacoustic image for

four strands of hair with offset in positions produced by 40 MHz non-focused

nanotube array transducer

VI. CONCLUSION

In summary, we have demonstrated high frequency ultrasonic

transducers using P(VDF-TrFE) nanotube array for the first

time at frequency as high as 108 MHz with maximum fractional

bandwidth of 126 % (-6 dB) and sensitivity 46 % higher than

P(VDF-TrFE) thin film. The scanning ultrasonic imaging with

nanotube array ultrasonic transducers shows a resolution up to

20 µm and photoacoustic image for hairs (dia. ~ 80 µm) can be

clearly resolved. The outstanding ultrasonic transducer

performances are attributed to the superior electromechanical

properties and acoustic impedance matching derived from the

unique characteristics of the nanotube array. The nano-

confinement effect during crystallization and eliminated in-

plane mechanical clamping significantly enhance the

electromechanical properties of our obtained piezoelectric

nanotubes. The hollow structure of nanotubes results in the

desired acoustic impedance matching with water-based

mediums. The implementation of piezoelectric nanotube array

in high frequency ultrasonic transducers and imaging with the

outstanding performance will inspire both fundamental

research and device development efforts on electromechanical

functional 1D nanostructures derived from bottom up approach.

VII. APPENDIX A

Fig. A1. 3-D drawing of the displacement data obtained with laser scanning

vibrometer under a 20 V amplitude sine-wave driving signal at 3 kHz, showing

effective d33 of 28 pm/V.

Fig. A2. Polarization-electric field hysteresis loop of the P(VDF-TrFE)

nanotube array.



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Fig. A3. Model and boundary conditions for the finite element analysis

(ANSYS) of (a) P(VDF-TrFE) nanotube and (b) P(VDF-TrFE) thin film. Static

pressure of 1000 Pa is applied aligned with the long axis of nanotube and

thickness direction of thin film. (upper) The strain induced by the static pressure

is 0.445 x 10-6 for nanotube and 0.215 x 10-6 for thin film. (lower) The effective

elastic compliance can be calculated for the respective structures.

Fig. A4. The transmission signal sensed by a 120 MHz PVDF film ultrasonic

transducer with P(VDF-TrFE) thin film and nanotube array ultrasonic

transducers as pulsers. The pressure wave generated by nanotube array is

measured to be 36 % lower than thin film. The maximum pressure a

piezoelectric material can generate is estimated through the piezoelectric

constitutive equation,

S3 = s33E T3 + d33E3 (A1)

where S3 is the strain, T3 is the stress and E3 is the electric field. When the

piezoelectric element is driven by an electric field, the maximum pressure

occurs when the piezo-induced strain is completely blocked, Pmax = T3S3=0

. So

Pmax is proportional to the ratio of d33 s33E⁄ under constant electric field. Hence,

the nanotube array with higher elastic compliance (s33E ) is predicted to generate

lower pressure than thin film.

Fig. A5. Pulse echo signal of focused nanotube array ultrasonic transducer with

varying distance between ultrasonic transducer and target. Peak at ~2.6 mm

indicates focal length of ~ 2.6 mm for the focused nanotube array ultrasonic

transducer.

Fig. A6. (Top)Scanning ultrasonic images and (Bottom) the profiles of the

images (red dotted line) from (a) 40 MHz focused nanotube array ultrasonic

transducer, (b) 108 MHz focused nanotube array ultrasonic transducer and (c)

120 MHz (nominal) focused PVDF film ultrasonic transducer.

VIII. APPENDIX B

A. Fabrication of nanotube array and thin film ultrasonic

transducers

The one-end sealed ordered anodized alumina membrane

with pore size 350 – 400 nm and depth of 4 µm and 2 µm was

fabricated using a two-step anodization method. For non-

focused ultrasonic transducer, the alumina membrane was

fabricated on a flat Al foil whereas curved Al foil was used for

focused ultrasonic transducer. A high purity Al foil was electro-

polished with perchloric acid to obtain a mirror-finish surface.

The two-step anodization was performed on the high purity Al

foil in phosphoric acid under a constant applied voltage of 180

V for subsequent use for producing nanotubes with outer

diameter of 350 nm. Nanotube with outer diameter of 70 nm

was fabricated using the same process but oxalic acid was used

as the anodization agent and at 40 V of the applied voltage. The

pores of ordered anodized alumina membrane were enlarged by

chemical etching in 5 wt% phosphoric acid solution to produce

the desired pore sizes.

A 10 wt% P(VDF-TrFE) (72/28) solution was prepared by

dissolving the P(VDF-TrFE) pellets in a mixed solvent of

dimethyl-formamide (DMF) and acetone. The P(VDF-TrFE)

solution was spin-coated on the open end of the ordered

anodized alumina membrane and dried at 100 °C. The sample

was heated to 220 °C to melt the polymer. The polymer melt

wetted the ordered anodized alumina membrane and filled into

the pores by capillary force. The deposition was repeated to

obtain the desired nanotube wall thickness. P(VDF-TrFE)

nanotube arrays were produced as the one-dimensional

piezoelectric nanostructures embedded in ordered anodized

alumina membrane, attached to a residual P(VDF-TrFE)

polymer film. A thin film ultrasonic transducer was fabricated

by spin-coating the 10 wt% P(VDF-TrFE) (72/28) solution as

mentioned above on a piece of electro-polished Al foil. The

process was repeated to obtain a P(VDF-TrFE) film of 30 µm.

The remaining fabrication process was same as nanotube array

transducer.



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A bottom electrode comprising a layer of chromium with

thickness of 5 nm and a layer of gold with thickness of 50 nm

was deposited on the side of the nanotube arrays with the

residue polymer film by e-beam evaporation. The bottom

electrode was connected to a female connector as housing

through an electrical wire using silver epoxy. An epoxy was

filled into the empty space of the housing to form a backing for

the piezoelectric nanotube arrays. Residual Al foil was removed

by chemical etching using copper chloride solution (in

hydrochloric acid). The nanotube arrays were exposed by

controlled chemical etching of the ordered anodized alumina

membrane in phosphoric acid solution. The top electrode

comprising a layer of chromium with thickness of 5 nm and a

layer of gold with thickness of 50 nm deposited on the exposed

end of the nanotube arrays by e-beam evaporation. The

nanotube arrays were poled by applying an electric field of 1000

kV/cm, twice the coercive field as shown in the hysteresis loop

in Fig. A2, across the top and bottom electrodes using a direct

current (D. C.) voltage source.

The polarization versus electric field (P-E) hysteresis loops

were measured with a standard ferroelectric testing unit

(Precision Premier II, Radiant Technology) connected to a high

voltage interface. The effective piezoelectric constant d33 was

measured with a laser scanning vibrometer (OFV- 3001-SF6,

PolyTech GmbH).

B. Numerical simulation

Numerical simulation was carried out by using Ansys

(version 15.0, Ansys Inc., Canonsburg, PA) software. The

piezoelectric element was modeled by using the SOLID226

coupled field element. The model for simulation is shown in

Fig. A3. Simulation of acoustic impedance using k-space

pseudospectral method was performed using k-Wave:

MATLAB toolbox.

C. Ultrasonic testing

In pulse-echo mode, the nanotube array ultrasonic transducer

was excited by drive voltage pulse with pulse width (FWHM)

of less than 6.5 ns and generated an ultrasonic wave pulse into

the surrounding water. A glass slide was used as target. The

distance between the ultrasonic transducer and target was 4 mm

away. The first reflected ultrasonic wave pulse from the surface

of glass slide propagated back to the same ultrasonic transducer

and the signal was recorded by the ultrasonic pulser/receiver

and displayed on the computer after converted into digital

signal by the A/D converter.

In transmission mode, a 30 MHz - commercial ultrasonic

transducer (Olympus IMS) was used as the ultrasonic pulser to

generate an ultrasonic pulse. A 40 MHz-nanotube array

ultrasonic transducer and a 30-MHz P(VDF-TrFE) thin film

ultrasonic transducer were used as sensors. The ultrasonic wave

pulse sensed by ultrasonic transducers was recorded by the

ultrasonic pulser/receiver and displayed on the computer after

converted into digital signal by the A/D converter.

The scanning ultrasonic imaging was performed by

mechanically scanning the sample with a focused nanotube

array ultrasonic transducer. The mechanical scanning speed for

the ultrasonic imaging was 10 mm/min, with acquisition rate of

80 Hz; hence the step size was approximately 2 µm. The

distance between the ultrasonic transducer and sample was

adjusted until the focus of the ultrasonic transducer was on

surface of a steel plate underneath of adhesive tape. Copper

wires with diameter of 20 µm were used as the imaging sample

for high resolution ultrasonic imaging. Real time envelope

detection was performed to obtain radio frequency signal for

the formation of the ultrasonic images.

The photoacoustic imaging was performed by mechanically

scanning the sample with a non-focused nanotube ultrasonic

transducer. The mechanical scanning speed is 5 mm/min, with

acquisition rate of 10 Hz; hence the step size was approximately

8 µm. The distance between the ultrasonic transducer and the

sample was kept at a constant distance of 8 mm. Nd-Yag solid

state laser with pulse duration of ~10 ns and wavelength of 700

nm was used as the light source to excite the ultrasound waves

on the imaging targets.

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`

Weng Heng Liew received his Bachelor of Engineering degree

in Materials Science and Engineering from National University

of Singapore in 2012. He is pursuing Ph.D degree in National

University of Singapore. Currently, he is also a specialist in the

Institute of Materials Research and Engineering (IMRE),

A*STAR, Singapore. His research interests include functional

materials such as ferroelectric and piezoelectric materials,

ultrasonic transducers for biomedical ultrasound and

photoacoustic imaging.

Kui Yao received the bachelor degree in electronics

engineering, master degree in technical physics, and the Ph.D

in electronic materials and devices. Currently, he is a Principal

Scientist, and the Program Manager for the material-critical

Sensors and Transducers Programme at IMRE, A*STAR. He is

also an Adjunct Professor in School of Material Science and

Engineering (MSE), Nanyang Technical University (NTU).

During 1998 - 1999, he worked in the Materials Research

Laboratory (MRL), The Pennsylvania State University, USA.

Previously, he was a postdoctoral fellow in Microelectronics

Center, NTU, during 1995-1997. His research areas cover smart

materials, particularly dielectric, piezoelectric and ferroelectric

materials, and the sensors and transducers enabled with these

materials, including their applications for structural and

condition monitoring, ultrasonic and photo-acoustic non-

destructive testing and diagnosis, noise and vibration

mitigation.

Shuting Chen received his Bachelor degree in Material

Science and Engineering and Master degree in Physical

Chemistry of Materials from Tongji University in 2002 and

2005, respectively. He received Ph.D degree in Mechanical

Engineering from National University of Singapore in 2010.

Currently, he is a research scientist in the Institute of Materials

Research and Engineering (IMRE), A*STAR, Singapore. Prior

joining IMRE in August 2013, he worked in Failure Analysis

Lab of GLOBALFOUNDRIES, Singapore. His research

interests are in the area of smart materials including dielectric,

piezoelectric and ferroelectric materials for sensors and

transducers applications.

Francis Eng Hock Tay is an Associate Professor with the

Department of Mechanical Engineering, Faculty of

Engineering, National University of Singapore. He received the

Ph.D. degree from the Massachusetts Institute of Technology

(MIT), Cambridge, in 1996. Dr. Tay was the founding director

of the Microsystems Technology Initiative (MSTI), and had

established the Microsystems Technology Specialization. He

had also served as the Technical Advisor, in the Micro and

Nano Systems Laboratory, Institute of Materials Research

Engineering (IMRE). His research areas including MEMS,

Biotechnology, Nanotechnology, Wearable Devices, Fall Onset

Detection, Vital Signs Monitoring, Body Sensor Network,

Rehabilitation and Scoliosis. He is also the Principal

Investigator for several projects under Agency for Science,

Technology and Research (A*STAR) and one project under

Qatar National Research Fund (QNRF).

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