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Applied Surface Science

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Piezoelectricity of picosecond laser-synthesized perovskite BaTiO3nanoparticlesInsung Choia,⁎, Su-Jin Leea, Jong Chan Kimb, Yeon-gyu Kimc, Dong Yeol Hyeonc,Kyong-Soo Hongd, Jeong Suha, Dongsig Shina, Hu Young Jeongb,e, Kwi-Il Parkc,⁎

a Laser Industrial Technology Research Group, Korea Institute of Machinery and Materials (KIMM), 48, Mieumsandan 5-ro 41beon-gil, Busan 46744, Republic of KoreabDepartment of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), 50, UNIST-gil, Ulsan 44919, Republic of Koreac School of Materials Science and Engineering, Kyungpook National University (KNU), 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Koread Busan Center, Korea Basic Science Institute (KBSI), 30, Gwahaksandan 1-ro 60beon-gil, Busan 46742, Republic of KoreaeUNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST), 50, UNIST-gil, Ulsan 44919, Republic of Korea

A R T I C L E I N F O

Keywords:Picosecond laserLaser ablation in liquidBaTiO3 nanoparticlesPiezoelectric

A B S T R A C T

Flexible energy harvesters based on piezoelectric nanomaterials have attracted great attention because theyenable a self-powered system that converts electric energy from ambient energy resources. To achieve per-ovskite-structured piezoelectric nanopowders, most studies have utilized multi-step chemical-based hydro-thermal reaction with repeat purification processes, which results in time-consuming production, low-yieldproduction, and the inevitable existence of second phase. As an alternative that addresses the drawbacks ofprevious synthesis methods, the ultrafast laser processing based on cold ablation is highly promising for na-nomaterial synthesis. In this work, we investigated the feasibility of synthesizing BaTiO3 nanoparticles by thepicosecond laser ablation of a bulk BaTiO3 target in an ethanol solvent. The picosecond laser-synthesized BaTiO3nanoparticles have a spherical shape with a size of ~300 nm and typical perovskite crystallinity with tetragonalphase. To investigate the piezoelectric response from BaTiO3 nanoparticles, we used a piezoresponse forcemicroscope and confirmed the piezoelectric charge constant of ~130 pm⋅V−1. The results indicate the viabilityof synthesizing piezoelectric ceramic nanoparticles by the laser ablation of a bulk target in a liquid environment.

1. Introduction

Energy harvesting technology based on piezoelectric effects hasattracted a great interest because it can be used to convert the usableelectrical energy from vibrational and mechanical deformations, whichare more accessible than other renewable energy resources [1,2]. Inparticular, flexible energy harvesters made of piezoelectric nanoma-terials, called nanogenerators (NGs), that harvest electricity from am-bient tiny movements created by natural sources or from human ac-tivities, such as finger motions, stretching, heartbeat, or eye blinking,have been developed by many researchers [3–10]. A variety of archi-tectures of NG have been steadily proposed to enhance electrical powerto demonstrate the operation of low-power devices, such as LEDs andLCD screens without external battery supplies [3–8]. Among the manykinds of NGs, a piezoelectric composite-based NG device realized byadopting inorganic piezoelectric nanoparticles and a polymeric matrix,which is called a nanocomposite generator (NCG), has shown a con-siderable potential for a high-output, large-area, and mechanically

stable/flexible piezoelectric energy harvester (f-PEH). Piezoelectriccomposites were produced by dispersing inherently high piezoelectricperovskite-structured ceramic nanomaterials [such as BaTiO3 (BTO),PbZrxTi1-xO3 (PZT), NaNbO3, KNbO3, or 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT)] inside the polydimethylsiloxane (PDMS)elastomer and subsequently spin-casted onto electrode-coated flexiblesubstrates [6,7,11–16]. To synthesize piezo-ceramic nanomaterials, theconventional hydrothermal reaction has been utilized, though it re-quires complex chemical-based synthesis steps and an additional pur-ification process [6,17,18].

As an alternative to the conventional synthesis method, the nano-particles can be simply produced by pulsed laser ablation of solid tar-gets in liquid environments, which is referred to as laser ablation inliquid (LAL) method [19–21]. The main advantages of LAL are that itprovides high purity of materials and a short production time withoutthe need for a long chemical reaction time or complex purificationsteps. Furthermore, the target material’s crystal structure and stoi-chiometry can be preserved in the nanoparticle colloids [22–24]. On

https://doi.org/10.1016/j.apsusc.2020.145614Received 13 November 2019; Received in revised form 16 January 2020; Accepted 31 January 2020

⁎ Corresponding authors.E-mail addresses: ichoi@kimm.re.kr (I. Choi), kipark@knu.ac.kr (K.-I. Park).

Applied Surface Science 511 (2020) 145614

Available online 01 February 20200169-4332/ © 2020 Elsevier B.V. All rights reserved.

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the other hand, the bottleneck of the method for industrial applicationshas been known as low productivity such as milligram-scale synthesisper hour. Some researchers have developed a system by adopting a li-quid flow chamber and a high-speed polygon scanner (500 m/s) toimprove the productivity of nanopowders, resulting in productivities ofgram-scale/hour [25–27]. Although many studies in the field of LALhave been published over the past few decades [19–21], perovskitestructured piezo-ceramics have not been studied to synthesize the na-nopowders and to characterize their piezoelectricity.

In this work, we have demonstrated the synthesis of perovskite BTOnanoparticles by the laser ablation of a bulk BTO target in an ethanolsolvent. An ultrafast laser (λ = 1030 nm, pulse duration = 1 ps) and agalvanometer scanner system were used with optimized experimentalconditions to avoid the thermal effect and retain the target material’sstoichiometric composition. Various analyses were carried out tocharacterize the material properties by UV–Vis spectrometry, X-raydiffraction (XRD), Raman spectroscopy, and high-resolution transmis-sion electron microscopy (HR-TEM). The obtained BTO nanoparticlesshowed a rounded shape with a size of ~300 nm and perovskite crys-tallinity with tetragonal phase. Moreover, we used piezoresponse forcemicroscopy (PFM) technique to explore the piezoelectricity of the laser-synthesized BTO nanoparticles. From the piezoelectric displacement ofalong to the applied external voltage, we confirmed that a BTO singlenanoparticle has a piezoelectric charge constant of ~130 pm⋅V−1. Ourresults provide the feasibility to synthesize the perovskite-structuredpiezoelectric nanoparticles by means of ultrafast laser materials pro-cessing.

2. Experimental

2.1. Laser ablation in liquid

The LAL experiments were performed using a picosecond lasersystem (AMPHOS 400) and its specifications are given in Table 1.Fig. 1a shows a schematic diagram of the laser ablation process which isapplied to the work piece (here, a bulk BTO target) in a liquid en-vironment by means of a two-axis galvanometer scanner head (RAY-LASE) with a 160 mm focal length of the f-theta focusing lens (JENO-PTIK). A computer-controlled X-Y-Z stage (AEROTECH) was used tomount the sample and for initial positioning of the experimental samplewithin the galvanometer scanning zone. All the experiments for thiswork were carried out with a pulse duration of 1 ps, a repetition rate of0.8 MHz, and an average power up to 51.2 W. The applied scan speedwas varied from 0.5 to 3 m/s over a fixed 9 mm × 9 mm multilineablation pattern, providing an interline distance of 50 μm.

2.2. Characterization methods

The optical absorption spectra were measured by UV–Vis spectro-metry (Agilent Cary 300). Transmittance data were obtained afterbackground subtraction in the range of 250–650 nm with a step size of0.2 nm; then, absorbance data were converted from the transmittancedata. The morphology of the synthesized BTO nanoparticles was

observed via field-emission scanning electron microscopy (FE-SEM,Hitachi SU8220). The particle size and size distributions were estimatedbased on the measured values. The crystal structure of the obtainedBTO nanoparticles were investigated via X-ray diffraction (XRD, RigakuD/Max-2500) with CuKα radiation (λ = 1.5406 Å) operating at 40 kVand 200 mA with a step size of 0.02° at 2° min−1 in a 2-theta range of15–60°. To comprehensively analyze the phase of the BTO nano-particles, micro-Raman measurements with a spot size of about 1 μmwere implemented at room temperature using Raman (WITecalpha300R). A 532 nm excitation laser with a five-second integrationtime per spectrum was focused on the BTO nanoparticles via a50 × objective lens. To structurally characterize the BTO nanoparticles,we acquired atomic resolution images using a high-resolution trans-mission electron microscope (HR-TEM, JEOL JEM-2100F) with a probeCs corrector operated at 200 kV. Fast fourier transform (FFT) patternswere obtained from the HR-TEM images to investigate the crystallinityof each sample. High-angle annular dark field (HAADF) scanning TEM(STEM) and an energy-dispersive X-ray spectroscopy (EDS) were uti-lized to analyze the chemical structure of materials. The piezoelectricresponse of the BTO nanoparticles was characterized by atomic forcemicroscopy (AFM, Park Systems NX20) with a conductive Pt tip. Toacquire the piezoelectric charge constant, d33 (the induced polarizationper unit stress applied in an out-of-plane direction), of the BTO nano-particles, displacement along the Z-axis when subjected to appliedvoltages ranging from 0 to 10 V was measured.

3. Results and discussion

Low laser fluences (pulse energy per unit area) in an ultrafast laserlead to cold ablation with negligible heat transfer to the surroundinglattice, and this enables precise ablation of materials without traces ofmelt [22,28]. This non-thermal ablation and accurate processing canalso be applied in liquids, which can even be inflammable. This featureallows laser ablation directly in organic solvents. Various synthesismechanisms of nanoparticles by LAL have been reported, depending ondifferent pulse durations [19–21]. In the case of picosecond laser ab-lation, a laser pulse interacts with a target material immersed in a li-quid, which brings about the formation of a plasma on the target sur-face. This plasma generates high temperature and high pressures, and ittransfers its energy to the liquid during its expansion. As a consequence,a layer of vapor is formed around the plasma volume [27]. The vaporlayer expands into cavitation bubbles and plasma quenching takesplace. Next, the plasma collapses and releases the nanoparticles intobubbles. Finally, the bubbles collapse and release the nanoparticles intoliquids [19–21]. The cavitation bubble is known that it can absorb,reflect or scatter subsequent laser pulses, thereby causes low productionof the nanoparticles. To obtain higher productivity of nanoparticleswith the latest high-power picosecond lasers, a method for spatial by-passing of the cavitation bubble has reported by consideration of laserprocessing parameters [25]. Recently, Streubel et al. developed an ex-perimental set up with a high-speed polygon scanner (500 m/s) to avoidthe cavitation bubble interaction. They then succeeded in pilot-scalesynthesis (4 g/h) of nanoparticles [26,27]. In this work, we focused onthe feasibility of production of perovskite piezoelectric nanoparticlesdue to limited experimental conditions with a galvanometer scannerspeed of ~5 m/s.

Fig. 1a shows a schematic of the experimental set up for LAL. In thisstudy, we used a 1 ps laser system (Amphos 400, 1030 nm wavelength)with an available power up to 400 W, and a repetition rate of between0.8 and 40 MHz. A bulk BTO target was prepared by a typical sinteringprocess. Commercial BaTiO3 powders (99%,

immersed in 10 mL ethanol for LAL experiments. The liquid thicknessabove the target surface was kept at 5 mm; thus, a laser beam wasirradiated on the bulk BTO target through a 5 mm thickness of ethanoland produced translucent colloids. The inset of Fig. 1a shows beam size(20 μm) and beam overlap (66–90%) depending on scan speed. Weinvestigated the optimal experimental conditions by means of laserpower, repetition rate, and available scanning speed of our lasersystem, as shown in Table 1. In order to avoid heat accumulation andpreserve stoichiometry of target material, a short pulse length (1 ps)and a low repetition rate (0.8 MHz) were utilized with low laser flu-ences per pulse ranged from 28 to 64µ J (correspond to laser powers of22.4 to 51.2 W). In addition, various scanning speeds were used to tryto avoid the change of chemical composition or amorphous phasetransition, although speed of galvanometer scanner leaded to the ca-vitation bubble interactions. The inset of Fig. 1b shows a colloid com-posed of BTO nanoparticles in a glass dish. The resultant was movedinto a vial for material characterizations. Vial (i) shows a more trans-lucent color meaning higher amounts of nanoparticles due to the lowerscan speed of 0.5 m/s in comparison to vial (ii) using a faster scan speedof 1.0 m/s. UV–Vis spectrometry was utilized to confirm the opticalabsorptions of vials (i) and (ii). The red and green lines correspond toscan speed of 0.5 and 1.0 m/s, respectively. Both lines show almostsame spectra and a bandgap is calculated as 3.19 eV (see details in Fig.S1). The value of bandgap energy is similar to that reported for bulkBTO (~3.2 eV) [29,30]. Fig. 1c indicates a wide range of nanoparticlesizes with irregular shapes and mostly average sizes of between 200 and300 nm. The crystal structure of the BTO nanoparticles synthesized byLAL was investigated by XRD. Fig. 1d compares the crystallinity of thenanoparticles (using scan speed of 1.0 m/s) on a glass substrate (blackline) with that of the bulk BTO target. The XRD data obtained fromlaser-ablated BTO nanoparticles clearly shows exactly the same as bulkdata and tetragonal phase (P4mm) with distinguishable peak separationat (200) that denotes asymmetric elongation along the c-axis (see de-tails in Fig. S2) [31].

Raman spectroscopy has been widely used as a powerful and non-destructive technique to confirm the crystal structure of materials.Fig. 2 compares the Raman spectra of the bulk BTO target with BTO

nanoparticles synthesized by variations of scan speed and laser power.At first, low power was used to avoid the thermal effect, and the scanspeed was changed from 0.5 to 3 m/s, as shown in Fig. 2a. All theRaman spectra show clear evidence of tetragonal phase in perovskiteBTO-based ceramics with sharp peaks at 306, 519, and 715 cm−1 at-tributed to E, B1(LO + TO), E, A1(TO) and E, A1(LO), respectively[32,33]. Interestingly, the Raman spectrum with a slow scan speed of0.5 m/s (red line) shows a shoulder peak between 610 and 640 cm−1,which corresponds to TiO2 crystal phase (see details in Fig. S3) [34,35].We assumed that a low scan speed might cause heat accumulation, andthe barium element was vapored from the BTO target. In addition, weinvestigated an effect on power increase with a relatively high scanspeed (3.0 m/s), as shown in Fig. 2b. The Raman spectra of the laser-synthesized BTO nanoparticles were similar to that of the bulk BTOtarget. However, the productivity of the nanoparticles was incrediblydecreased by increasing the laser power due to bigger cavitation bubbleinteractions. From our study, we believe that a high-speed polygonscanner is required for mass production up to gram scale, for applica-tion to energy harvesting devices. According to the analysis results fromRaman and XRD, BTO nanoparticles can be produced by the ultrafastlaser ablation of a bulk BTO target in a liquid environment, while re-taining the original crystal structure of the bulk target.

To better understand the crystal structure and chemical compositionof the BTO nanoparticles, the HR-TEM technique and EDS analysis wereperformed, as shown in Fig. 3. Fig. 3a shows bright-field (i) and a high-resolution (ii) TEM images of a BTO nanoparticle on a copper TEM-grid,synthesized by laser power of 22.4 W with a scan speed of 1.0 m/s. TheFFT pattern (Fig. 3a-iii) of the white-dotted area in (Fig. 3a-ii) corre-spond to (0 1 1̄), (2 0 1), and (2 1 0) reflections along the zone axis[1̄22], thereby demonstrating the tetragonal structure of the BTO. Tocharacterize the stoichiometric composition of BTO nanoparticle, STEMand EDS mapping were carried out, as shown in Fig. 3b. All the ele-ments in the BTO nanoparticle were visualized in Fig. 3b via the EDSmapping and they were well distributed throughout individual parti-cles. Fig. 3c shows EDS spectrum between 0 and 10 keV and the atomicpercentage of each element (Ba, Ti, and O). The obtained data from24.43%, 19.72%, and 55.85% which correspond to Ba, Ti, and O

Fig. 1. Synthesis of BaTiO3 nano-particles by laser ablation in liquid. (a)Schematic of laser ablation system withpicosecond laser, galvanometerscanner, and f-theta lens. Inset de-scribes laser beam size and overlaps bylateral scan. (b) UV–visible spectra ofBaTiO3 nanoparticles (BTO NPs) bylaser ablation of bulk BaTiO3 target inethanol. Red (i) and green (ii) linescorrespond to scan speed of 0.5 and1.0 m/s. Inset photos show BaTiO3 na-noparticles colloid in glass dish (left)after laser irradiation and two vials(right, i and ii) containing the colloidsfor material characterization. (c) SEMimage of laser-synthesized BaTiO3 na-noparticles. (d) X-ray diffraction pat-terns of the BaTiO3 nanoparticles onglass substrate (black line) and bulkBaTiO3 target (red line) as a reference.(For interpretation of the references tocolour in this figure legend, the readeris referred to the web version of thisarticle.)

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element, respectively, almost equal the atomic ratio of 1:1:3 (Ba:Ti:O).We also characterized the crystal structure of small BTO nanoparticles,as shown in Fig. S4. The edges of the BTO nanoparticles were in-vestigated to avoid the disturbance caused by agglomeration of BTOnanoparticles. The FFT pattern clearly corresponds to (0 1̄ 0), (1 1̄ 0),and (1 0 0) reflections along the zone axis [0 0 1], thereby proving thetetragonal phase of the BTO.

On the other hand, we rarely detected TiO2 nanoparticles by a scanspeed of 0.5 m/s and power of 22.4 W, as shown in Fig. S5. The FFTpattern demonstrated the tetragonal structure (rutile) of the TiO2 ma-terial. This observation of the TiO2 nanoparticles is in good agreementwith the Raman data, as shown in Fig. 2a. All the elements in TiO2 werevisualized via EDS mapping and they were well distributed throughindividual particles. The atomic ratio of the TiO2 nanoparticle is almost

1:2, which shows good agreement with the stoichiometric compositionof TiO2. From the observation of the TiO2 nanoparticles, we assumedthat a low scan speed might cause heat accumulation and vaporizationof the barium element.

To characterize the energy conversion efficiency of piezoelectricmaterials, we have determined the piezoelectric charge constant (dij),where the i and j subscripts are the directions of the generated dipolesand the introduced stress, respectively. The dij, defined by the re-lationship between the mechanical deformation and the producedcharge, represents the output performance of piezoelectric materialswhich can be converted from external forces. In this study, the PFMtechnique was used by means of the modified AFM tool to explore thedij of individual BTO piezoelectric nanoparticles. When the applied biasvoltage was applied to a single BTO nanoparticle by a Pt tip mounted-

Fig. 2. Raman spectra of the BaTiO3nanoparticles by laser ablation in li-quid. (a) Raman spectra on the BaTiO3nanoparticles corresponding to powerof 22.4 W with different scan speeds of0.5 m/s (red line), 1.0 m/s (green line),and 3.0 m/s (blue line). A black lineshows a spectrum of bulk BaTiO3 targetas a reference. (b) Raman spectra on theBaTiO3 nanoparticles corresponding topower variations with 27.2 W (blackline), 32.0 W (red line), 41.6 W (greenline), and 51.2 W (blue line). Scanspeeds were fixed to 3.0 m/s. (For in-terpretation of the references to colourin this figure legend, the reader is re-ferred to the web version of this article.)

Fig. 3. TEM and EDS analysis results ofthe BaTiO3 nanoparticles by laser abla-tion in liquid. (a) Bright-field TEMimage (i) of the BaTiO3 nanoparticle onTEM-grid, synthesized by power of22.4 W and scan speed of 1.0 m/s. TheMagnified HR-TEM image (ii) of theedge of the image (i), corresponding towhite-dotted area. The FFT pattern (iii)indicates the crystal orientation of thewhite-dotted area in (ii). (b) HAADFSTEM image of the BaTiO3 nanoparticlein (a) and EDS mapping of Ba, Ti, andO, respectively. (c) EDS spectrum andelemental atomic percent of the BaTiO3nanoparticle in (a).

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cantilever, the piezoelectric nanoparticle was stretched along the di-rection of the applied electric field in response to the piezoelectric ef-fect, as shown in Fig. 4a; simultaneously, we obtained a plot of thepiezo-response displacement as a function of the applied voltage (seeFig. 4b). The slope of the applied voltage to the displacement in a graphcan be obtained from the least squared line by the method of leastsquares. This slope that indicated the d33 of unpoled BTO single na-noparticle is ~130 pm⋅V−1, which is a reasonable constant in com-parison to one that was previously reported [4,36,37]. Table 2 showsthe d33, standard error, and coefficient of determination (R2) from tenpoints measured from the individual BTO nanoparticles.

4. Conclusions

In summary, our experimental results indicate the possibility ofproducing perovskite BaTiO3 nanoparticles by the picosecond laserablation of a bulk BaTiO3 target in aqueous media. This method pro-vides high purity of nanomaterials and short production time withoutthe need for a long chemical reaction time and purification steps. Avariety of analysis methods using UV–Vis spectrometry, XRD, Raman,TEM, and EDS were performed on laser-synthesized piezoelectric na-noparticles, and the results showed a BaTiO3 crystal structure withtetragonal phase. In addition to material characterizations, piezo-response measurements were carried out to investigate the piezo-electricity, resulting in ~130 pm⋅V−1 for an unpoled BaTiO3 singlenanoparticle. Therefore, this study proved a feasibility for nanoparticlesynthesis of ternary perovskite materials by laser ablation in liquid(LAL). To apply the piezoelectric nanoparticles synthesized by LALmethod to energy harvesting device applications, we believe that both ahigh-speed polygon scanner (~500 m/s) and a liquid flow chambersystem are essential for mass production, as demonstrated by previousstudies [26,27]. This work is expected to provide potential

opportunities for application to a simple production method of otherpiezoelectric nanopowders, such as quaternary and quinary perovskitematerials, as well as engineering/electronic ceramic nanomaterials.

Author contribution

I. Choi, D. Shin, and K.-I. Park conceptualized the idea of this study.S.-J. Lee, J. Suh, and D. Shin supervised laser experiments and sup-ported resources for experimental materials. I. Choi, J. C. Kim, Y.-G.Kim, D. Y. Hyeon, K.-S. Hong, and H. Y. Jeong played a role in datacuration. I. Choi and K.-I. Park wrote an original draft and completedthe revision work.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgements

This work was supported by Korea Institute of Machinery andMaterials of South Korea (NK222D). This research was also supportedby Basic Science Research Program through the National ResearchFoundation of South Korea (NRF) funded by the Ministry of Education(NRF-2019R1I1A2A01057073) and the Ministry of Science and ICT(NRF-2018R1A4A1022260).

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2020.145614.

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Piezoelectricity of picosecond laser-synthesized perovskite BaTiO3 nanoparticlesIntroductionExperimentalLaser ablation in liquidCharacterization methods

Results and discussionConclusionsAuthor contributionmk:H1_8AcknowledgementsSupplementary materialReferences