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