Nano Energy 46 (2018) 314–321
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Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Communication
Multilayered carbon nanotube yarn based optoacoustic transducer with high energy conversion efficiency for ultrasound application
T
Zeyu Chena,b,1, Yue Wua,1, Yang Yangc,1, Jiapu Lia, Benshuai Xiec, Xiangjia Lic, Shuang Leia, ⁎ Jun Ou-Yanga, Xiaofei Yanga, Qifa Zhoub, Benpeng Zhua,d, a
School of Optical and Electronic Information, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China Department of Biomedical Engineering, University of Southern California, Los Angeles, CA 90089, USA c Daniel J. Epstein Department of Industrial and Systems Engineering, University of Southern California, Los Angeles, CA 90089, USA d Engineering Research Center for Functional Ceramics, Ministry of Education, Huazhong University of Science and Technology, Wuhan, 430074, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: CNT yarn Optoacoustic transducer Localized surface plasmon resonance Particle manipulation
Carbon Nanotube assemblies are currently of considerable interest for their properties of high-packing-density and aligned configurations, which offer a promising route to achieve higher strength and conductivity in macroscopic materials. While most researches have focused on their mechanical and electrical properties, recently, their light absorption and thermal conductivity have attracted more and more attention. This work introduces a novel freestanding optoacoustic transducer using continuous multilayered carbon nanotube yarns, gold nanoparticles and elastomeric polymer, which can be used for efficient conversion from laser energy to acoustic power and generating high acoustic pressure (~33.6 MPa) without focusing. The calculated energy conversion efficiency is as high as 2.74 × 10−2. Such excellent performances could be attributed to its freestanding structure and the enhanced optical absorption due to localized surface plasmon resonance (LSPR) caused by gold nanoparticles. The laser-generated ultrasound has been experimentally demonstrated to be capable of manipulating micro particles (50 µm) in a transparent channel.
1. Introduction The applications of ultrasound are far reaching, ranging from medical imaging, flaw detection to particle manipulation [1–3]. The traditional material to generate ultrasound is the piezoelectric materials that convert electric signal to compressive/tensile stresses, or vice versa [4–7]. Besides, researchers are also interested in generating ultrasound by other materials except the piezoelectric. Since the early 1980s there has been considerable interest in the production of ultrasound by irradiation of a solid with a laser pulse [8]. This optoacoustic transducer converts absorbed laser or modulated light into heat energy and temporally changes the temperatures at the loci where radiation is absorbed. This process results in an expansion and contraction following the temperature changes, which are translated to acoustic waves. The laser-ultrasound transduction structure usually consists of two kinds of materials: one for laser (light) absorption and the other for thermal expansion. Actually, high acoustic pressure is expected in medical application such as drug delivery and ultrasound therapy [9–11]. In order to obtain high performance and high-pressure acoustic pulse from
⁎
1
optoacoustic transducer, many efforts have been made on the absorbing materials with a high light absorption ratio and expanding layer or matrix with a high thermal expansion coefficient. Polydimethylsiloxane (PDMS), with a thermal coefficient of volume expansion (α = 0.92 × 10−3K−1) higher than metal, water and other polymers, is the most popular thermal expanding layer [12–15]. The main lightabsorbing materials involve gold (metal) and carbons. In 2006, Hou et al. reported a 1.5 MPa high frequency ultrasound generated by 2D gold nanostructure/PDMS [16]. In 2014, Colchester et al. achieved 4.5 MPa acoustic pressures using CNT-PDMS composite [17]. In 2015, Chang et al. successfully generated ultrasound with 12.15 MPa pressure using the carbon nanofibers/PDMS composite [14]. Carbons, especially CNT and its Assemblies, have been shown to possess the excellent lightabsorption and thermal conductivity compared with any known material, which is consider as the suitable absorption materials for optoacoustic transducer. In recent years, novel nanotube buckypaper, arrays, sheets and yarns have been developed and these superfiber materials have enormous potentials for optoacoustic transducer [18–20]. Besides, Baac et al. fabricated gold layer-coated CNT-PDMS into concave shape
Corresponding author. E-mail address:
[email protected] (B. Zhu). Contribute equally to this paper.
https://doi.org/10.1016/j.nanoen.2018.02.006 Received 29 November 2017; Received in revised form 19 January 2018; Accepted 1 February 2018 Available online 07 February 2018 2211-2855/ © 2018 Elsevier Ltd. All rights reserved.
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Au nanoparticle deposition. Fig. 1d shows the photograph of Tape I transducer. According to SEM across-sectional image of this freestanding structure (shown in Fig. 1e), PDMS is completely infiltrated among the layered CNT yarn, meanwhile, the obtained transducer has a soft backing layer PDMS with the thickness around 40 µm. Fig. 2 displays the top & bottom view (a) and side view (b) of the transducer and the direction of laser using schematics.
(focused shape) and generated 50 MPa in peak positive. [13] However, they did not clarify the enhancing mechanism of gold layer and concave shape, respectively. So the effect coming from gold layer deserves more attention. In this study, we demonstrate a novel optoacoustic transducer, which can produce high optoacoustic pressure (~33.6 MPa) and optoacoustic energy conversion efficiency of 2.74 × 10−2 in planar shape. We compare the acoustic pressure and energy conversion efficiency of this transducer with those in literatures. The high performance of our transducer can be attributed to a uniquely designed structure made of continuous multilayered carbon nanotube yarns-gold particles-polymer composites. We simulate the energy distribution around the gold particle in optoacoustic transducer and theoretically analyze the lasergenerated ultrasound to unveil the corresponding enhancement mechanisms of optoacoustic effect. Such high intensity ultrasound can be used for microfluidic flow control. The acoustic performance of the optoacoustic transducer is characterized in a transparent channel filled with micro particles (50 µm). Remarkably, it is shown that the lasergenerated ultrasound produces powerful force that can manipulate the particles.
2.2. Experimental configurations for acoustic characterization
2. Experimental section
Fig. 3 shows the experimental schematic used for generation and characterization of the laser-induced ultrasound. A 6 ns pulsed laser with 532 nm wavelength (Lapa-80, Bejing Lei Bao Optoelectronics Technology Co., Ltd) was used with a repetition rate of 20 Hz. The initial aperture size of the laser beam is 5 mm. The laser beam penetrated through a transparent wall of the water tank and shined on the optoacoustic transducer with multilayered CNT yarn. A piezoelectric hydrophone with 0.2 mm diameter (Precision Acoustic, UK) was used to detect the acoustic wave. The signals were then recorded by an oscilloscope (TDS, 2024B, Tektronix, USA). During the acoustic characterization, the hydrophone was initially mounted close to the optoacoustic transducer (around 5 mm) to collect the ultrasound signals.
2.1. Transducer fabrication
3. Results and discussion
The continuous yarn of CNTs with multiple-layer structure was fabricated by chemical vapor deposition (CVD) spinning process. Firstly, CNTs were produced in the gas flows from the CVD reaction with a mixed acetone and ethanol carbon source. Then monolayers of CNTs were assembled concentrically in seamless tubules along the yarn axis to form the CNT yarn. [21]Then we fabricate two types of transducer with the layered CNT yarn, which were labeled as type Ⅰ and type Ⅱ, respectively. For type Ⅰ, the gold particles (10 nm) were deposited on the freestanding multilayered CNT yarn by using sputtering system (Emitech k550x, vacuum: 10−2 Pa, target-substrate distance: 5 cm, sputtering time: 45 s), while no gold particle deposition was carried out for type Ⅱ. Then, PDMS was spin-coated over the CNT yarn at 4500 r.p.m. for 400 s, and then cured at 120 °C for 30 min. Finally, type Ⅰ and type Ⅱ transducers are of PDMS/Au-CNT yarn-PMDS structure and PDMS/CNT yarn-PDMS structure, respectively. In type Ⅰ transducer, the CNT yarn and PDMS formed a layered structure with gold particle deposited on the interface (Fig. 1a). This structure resulted in high energy conversion efficiency of our optoacoustic transducer, which will be discussed below. Fig. 1b displays scanning electron microscope image of the side view of multilayer CNT yarn with a thickness about 5 µm. Transmission electron microscopy image of single CNT wire shown in Fig. 1c. This can be obtained by ultrasonic dispersion of CNT yarn with
As shown in Fig. 4a, compared with pure CNT yarn, CNT yarn with Au nanoparticles exhibits superior light-absorbing behavior. When the incident light's wavelength is 532 nm, the absorbance of Au-CNT yarn is 1.5 times larger than that of pure CNT yarn. Using the type Ⅰ and type Ⅱ optoacoustic transducer, strong acoustic pressure was observed when the distance between transducer and hydrophone was about 5 mm. Experimental acoustic waveforms and spectrum of the laser-generated ultrasounds are shown in Fig. 4b and c. When the laser energy was 45 mJ/cm2, the peak positive pressure of the typeⅠ transducer corresponds to ~ 33.6 MPa and the negative pressure is ~ 10 MPa. With the same laser energy, the measured peak positive pressure and negative pressure of type Ⅱ transducer is of ~ 19 MPa and ~2 MPa respectively. As for Type I transducer, the central frequency is 11.8 MHz and bandwidth at − 6 dB is 179%. Meanwhile, the Type II transducer has a slightly lower central frequency (9.3 MHz) but a broader bandwidth (192%). Fig. 4d and its inset display the positive pressure and negative pressure of two types of transducer under different energy density. It is easy to find that the laser-generated ultrasound pressure increases with the increasing laser energy density. When energy density is the same, the type I transducer will produce much higher acoustic pressure than type II. Table 1 compares acoustic pressure, energy density between previous literature and our work. This displays that carbon materials Fig. 1. (a) SEM image of the top view multilayered CNT yarn deposited with gold particles. (b) Scanning electron microscope image of the side view of multilayer CNT yarn. (c) TEM image of single CNT wire with Au nanoparticles. (d) The photo of Tape I transducer with freestanding structure. (e) The SEM cross-sectional image of PDMS/Au-CNT Yarn-PDMS structure.
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Fig. 2. Schematic of optoacoustic transducer's side view and the direction of laser direction (a), top and bottom view (b).
Fig. 3. Experimental schematic for generation and characterization of the laser-induced ultrasound.
and type Ⅱ transducer are 2.74 × 10−2, and 8.6 × 10−3, respectively. Both of them are higher than the η of 4.41 × 10−3, reported in Chang's work [15]. These results further prove that, carbon nanotube yarns have great potential to serve as promising transducer for laser-generated ultrasound. Besides, Au nanoparticles in the CNT yarn-PDMS composite significantly intensify the optoacoustic effect. In order to fully understand the high acoustic pressure and energy conversion efficiency coming from the obtained optoacoustic transducer, the theoretical analysis was conducted. Actually, our transducer has two layers: pure PDMS layer and Au-CNT yarn-PDMS layer, while Au nanoparticles are mainly located at the interface (shown in Fig. 5a). The temperature variation on the interface was simulated by COMSOL Multiphysics. Under the same laser energy density, Fig. 5b shows the PDMS/Au-CNT yarn-PDMS structure with a 10 nm Au particle at the interface, while Fig. 5c is the PDMS/CNT yarn-PDMS structure. The color bar presents the normalized temperature. The results show that gold nanoparticle leads to significant heating of the nanostructure itself and its immediate environment, which results in a deeper penetration of heat energy in composite through CNT yarn. This phenomenon can be attributed to photoemission process [24], in which the energy of incident photons is used to eject an electron from a metal surface. The excitation of an electron in metal results in two types of carrier: an electron and a hole. After a rapid relaxation processes, the carrier's energy is converted into heat. Planar metal layer surfaces reflect most of the incident light, and light absorption is not very efficient. In metallic nanostructures, light absorption can be further enhanced by exciting localized surface plasmon resonance (LSPR). This produces an antenna effect resulting in light collection and heat concentration from an area that is larger than its physical size (shown in Fig. 5d).
such as CNT and CNF have obvious advantages over metal materials in generating high acoustic pressure. And the multilayered CNT yarn used in our study is an efficient absorbing material for high amplitude lasergenerated ultrasound. A further comparison was then made on the optoacoustic conversion efficiency. The optoacoustic conversion efficiency (K ) was introduced by Baac et al. [13]. It is defined by the energy conversion from the laser light to the acoustic pressure, which can be expressed as
K=
1 P (t ) dt T T 1 I (t ) dt T T
∫ ∫
(1)
where I (t ) is the average optical intensity and P (t ) is the average acoustic pressure during the period T . Baac et al. reported the K value of the CNT/Au/PDMS films on pre-focused glass substrate as 1.4 × 10−3 Pa/(W/m2). Our PDMS/CNT yarn-PDMS transducer (type Ⅱ) has the K value of 3.46 × 10−3 Pa/(W/m2). The K value of our PDMS/Au-CNT yarn-PDMS transducer (type Ⅰ) is 7.62 × 10−3 Pa/(W/ m2) and it is higher than that of Baac's focused optoacoustic transducer. The efficiency of the optoacoustic wave generation was introduced by Chang et. al [15]. It can be described by the following equation:
η=
Ea =
Ea Eoptical A ρc
∫0
(2) ∞
P 2 (t ) dt
(3)
where P (t ) is the acoustic pressure, A is the laser facula area, ρ is the water's density, c is ultrasound speed in water and Eoptical is pulse laser’ energy and Ea is the energy of the acoustic signal. The η of our type Ⅰ 316
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Fig. 4. (a) Absorption of CNT yarn with Au structure and pure CNT yarn. Acoustic waveforms (b) and spectrum (c) of two types of transducers. (d) Peak pressure of two types of transducer under different energy density.
Table 1 Acoustic pressure and experimental parameters comparison.
∇2 Pi −
Materials
Peak Pressure
Energy Density
Shape
PDMS/Au-CNT yarn-PDMS PDMS/CNT yarn-PDMS Glass/CSNP/PDMS [15] Fiber/MWCNT/gel [22] Glass/CNT/Au/PDMS [13] Glass/CNF/PDMS [14] Glass/2D gold structure [16] Glass/rGO/PDMS [23]
33.6 MPa 19 MPa 7.5 MPa 21.5 MPa 22 MPa 12.15 MPa 1.5 MPa 5.5 MPa
45 mJ/cm2 45 mJ/cm2 43.28 mJ/cm2 31.1 mJ/cm2 42.4 mJ/cm2 12.15 mJ/cm2 20.37 mJ/cm2 25 mJ/cm2
Planar Planar Planar Fiber Focused Planar Planar Planar
(5)
where P is the pressure, βT is the thermal volume expansion constant, and c is the sound speed in the medium. The thermal source in the 2nd medium has the form of the equation
S2 = βI0 e−βx e jωt
(6)
where β is the light absorption coefficient, I0 is the intensity of the incident light, ω is the angular frequency, and x is the coordinate. Based on the Eqs. (4)–(7) above and the boundary conditions (supporting information), the pressure in the water can be expressed as
P1 (x , h) = D1 e jwt − k1 (x − h)
For our optoacoustic transducer, pure PDMS is considered as the soft backing layer, while Au-CNT yarn (or pure CNT yarn) absorbs light as the thermal energy and transmits it into the surrounding PDMS immediately. According to Ref [25,26], the approximated diffusion equation of the thermal wave is
∂2Ti 1 ∂Si = ∂t2 ρi CPi ∂t
1 ∂2Pi ∂ 2T = −ρi βTi 2i ci 2 ∂t 2 ∂t
D1 =
(7)
Z2 βT2 βI0 α2 2(β + k2 R23) − (β + k2)(1 + R23) e (k2− β ) h − (β − k2)(1 − R23) e−(k2+ β ) h (1 + R21)(1 + R23) e k2 h − (1 − R21)(1 − R23) e−k2 h β 2 − k 22 κ2
(8) Unlike other optoacoustic transducers reported in the previous researches using glass as backing material, our transducer owns a soft backing layer (pure PDMS). This is the main reason why our transducer can produce a larger negative pressure. The carbon material we used is CNT yarn that has a freestanding structure, indicating that no glass substrate is needed. Therefore, the influence on optoacoustic signal brought by hard backing can be eliminated. As shown in Fig. 6a and b, for both the Type I and Type II transducers, the experiment results are in good agreement with the simulation ones. The manipulation of small amounts of liquids and particles has applications ranging from biomedical device to drug delivery. Ultrasound-driven manipulation is widely used in microfluidic channel. Direct light-driven manipulation is of particular interest because light
(4)
Where ρ is the density and CP is the thermal capacity which equals CPi = κi/ ρi αi . κ, α, t, T and S are the thermal conductivity, the thermal diffusivity, the time, the temperature, and an arbitrary thermal source, respectively. Subscript I denotes the layer numbers as shown in the Fig. 5a. The Water, Au-CNT yarn-PDMS (or CNT yarn-PDMS) composite layer and pure PDMS layer are labeled as 1, 2 and 3, consecutively. The thickness of the composite layer is h. In the media, the thermal-mechanical coupled equations can be given below [27], 317
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Fig. 5. (a)The schematic of the optoacoustic transducer. Temperature variation on the interface of type Ⅰ (b) and type Ⅱ (c) transducers. (d)Schematic of localized surface plasmon resonance.
of the optoacoustic control system. An optoacoustic transducer with PDMS/Au-CNT yarn-PDMS structure is attached on the one end of a transparent channel (PMMA, outside diameter 10 mm; inner diameter 8 mm). A 6 ns pulsed laser with 532 nm wavelength and 20 mJ/pulse
can provide contactless spatial and temporal control. However, existing light-driven technologies are limited by strongly resisted by the effect of contact-line pinning [28]. Here we report a strategy to manipulate micro particles by optoacoustic transducer. Fig. 7 shows the schematic
Fig. 6. Comparison of simulation and experiment results for Type I (a) and Type II (b).
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Fig. 7. Schematic of the (a) optoacoustic control system and (b) Movement of micro particles under laser-induced ultrasound.
Fig. 8. Optical images of particles manipulation process at different times.
under the same intensity of laser (supporting video 2). These promising results prove that the optoacoustic transducer is able to control liquids and particles over a long distance with controllable direction. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2018.02.006.
laser energy was employed. Micro particles with 50 µm diameter were added in the transparent channel through a window on the channel to make sure the particles were close to the end with optoacoustic transducer. In order to capture the moving of particles, a camera was fixed beside the channel. A video (supporting video 1) recorded the optoacoustic manipulation process. Fig. 8 displays the optical images of the channel at different time. Fig. 8a is the transparent channel without laser (0–4 s). The laser was turned on at 4 s. Then the optoacoustic transducer absorbed the laser energy, and generates ultrasound, which can push the micro particles. The following Figures show a micro particle cluster manipulated by the laser-generated ultrasound at different time (8 s, 10 s, 12 s, 14 s). For comparison, we used a transparent film instead of the PDMS/Au-CNT yarn-PDMS structure. These micrometer scale particles would not be controlled if there is no transducer
4. Conclusion In summary, we fabricated a novel optoacoustic transducer using multilayered carbon nanotube yarn. Through a specifically designed PDMS/Au-CNT yarn-PDMS structure, the optoacoustic transducer displays high acoustic pressure and energy conversion efficiency. The temperature variation on the interface was simulated to understand the enhancement mechanisms of gold nanoparticles. For the laser-
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generated ultrasound, the experiment results were very consistent with the theoretical analysis. The optoaousctic transducer (Type I) was successfully utilized to manipulate micro particles along certain direction. These results indicate that the superfiber materials such as CNT yarn are promising absorber for optoacoustic effect, and the metal nanoparticles as well as soft backing layer can greatly increase the acoustic pressure and conversion efficiency. High intensity ultrasound enabled by the novel optoacoustic transducers and device can bring us tremendous possibilities in applications such as microfluidic flow control system and drug delivery.
[17]
[18] [19]
[20]
Acknowledgements
[21]
This work was supported by the Natural Science Foundation of China (Grant no. 11774117 and 11574096), Natural Science Foundation Instrument Project of China (Grant no. 81727805), the Fundamental Research Funds for the Central Universities (2016YXZD038) and the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and astronautics) (Grant No. MCMS-0317G01). We thank Dr. Ziyu Wang and Dr. Dongdong Li for their help in SEM test and discussion. We also thank the Analytical and Testing Center of Huazhong University of Science & Technology
[22]
[23]
[24] [25] [26]
Appendix A. Supporting information
[27]
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen.2018.02.006.
[28]
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Zeyu Chen received a B.E. degree from Central South University, China, in 2012. He is currently a Postdoc in the Department of Biomedical Engineering at the University of Southern California (USC) under the direction of Dr. Qifa Zhou and Dr. K. Kirk Shung. He is conducting his research in the NIH Ultrasonic Transducer Resource Center (UTRC) on ultrasonic transducer fabrication and additive manufacturing of functional materials.
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Yue Wu received the Bachelor degree in electronic science and technology from Hunan University, China, in 2015. She is currently studying for the master degree in microelectronics and solid-state electronics at Huazhong University of Science and Technology, Wuhan, China. Her research interest is optoacoustic transducers design and fabrication.
Dr. Yang Yang is a Postdoctoral Research Associate in the Department of Industrial and Systems Engineering in University of Southern California. He received the Ph.D. Degree from School of Physics and Technology, Wuhan University in 2015. He is a joint Ph.D student in University of California, Los Angeles in 2014 and visiting researcher in University of Southern California in 2015. His research work concentrates on 3D printing of bioinspired structures and functional ceramics, self healing polymers, flexible sensors as well as high dielectric polymer-based nanocomposites.
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Z. Chen et al. Jiapu Li received his Master's degree from the Optics and Electronic Information at Huazhong University of Science and Technology in 2016. He is currently pursuing his Ph.D. degree at Huazhong University of Science and Technology. His research interests mainly focus on ultrasound transducer for medical applications.
Jun Ou-Yang received his Ph.D. degree from the Department of Electronic Science and Technology at Huazhong University of Science and Technology, China, in 2012. He is currently a Lecturer in the School of Optical and Electronic Information at Huazhong University of Science and Technology. His research interests focus on smart materials and sensors.
Benshuai Xie received a B.S. degree from Shanghai Jiao Tong University, China, in 2015. He finished his graduate study as a master student at University of Southern California in December 2017 and currently work as a research intern at the imaging core of Cedars-Sinai Medical Center. His research concentrates on new generation additive manufacturing process and its application on functional structure fabrication.
Xiaofei Yang received his Ph.D. degree from the Department of Electronic Science and Technology at Huazhong University of Science and Technology, China, in 1999. He is currently a Professor in the School of Optical and Electronic Information at Huazhong University of Science and Technology. His research interests focus on smart materials and intelligent system.
Xiangjia Li received a M.S. degree from Nanjing University of Aeronautics and Astronautics, China, in 2014. She is currently a PH.D. candidate in the Daniel J. Epstein Department of Industrial & Systems Engineering at University of Southern California (USC) under the direction of Dr. Yong Chen. She conducts research on development of multi-scale and multi-material additive manufacturing of functional materials in the Center for Advanced Manufacturing.
Qifa Zhou received his Ph.D. degree from the Department of Electronic Materials and Engineering of Xi'an Jiaotong University, China in 1993. He is currently Professor in the Department of Ophthalmology as well as the Department of Biomedical Engineering at USC. He is also one of the leading principle investigators at the NIH Resource Center on Medical Transducer Technology. He has published more than 200 peer-reviewed articles in journals including Nature Medicine and Progress in Materials Science, and is a fellow of the International Society for Optics and Photonics (SPIE) and the American Institute for Medical and Biological Engineering (AIMBE).
Shuang Lei is currently a M. S. candidate in School of Optical and Electronic Information at Huazhong University of Science and Technology. His major is microelectronic engineering and his present research interest focuses on the theoretic simulation for medical ultrasound transducer.
Benpeng Zhu was a joint Ph.D. student of the Department of Biomedical Engineering at University of Southern California, Los Angeles, CA, and School of Physics and Technology at Wuhan University, China, and received his Ph.D. degree in 2009. He is currently Professor in the School of Optical and Electronic Information at Huazhong University of Science and Technology. His research interests include the development of ultrasound transducer, medical imaging and microparticle manipulation.
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