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Inverse size-dependence of piezoelectricity in single BaTiO3 nanoparticles

Sung-Dae Kima,1, Geon-Tae Hwanga,1, Kyung Songa,1, Chang Kyu Jeongb, Kwi-Il Parkc,Jinhyuk Jangf, Kwang-Ho Kimd,⁎, Jungho Ryue,⁎, Si-Young Choif,⁎

a Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of KoreabDivision of Advanced Materials Engineering, Chonbuk National University, Jeonju 54896, South Koreac School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, South Koread School of Materials Science and Engineering, Pusan National University, Busan 46241, South Koreae School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, South KoreafDepartment of Materials Science and Engineering, POSTECH, Pohang 37673, South Korea

A R T I C L E I N F O

Keywords:FerroelectricSize effectPiezoelectricNanogeneratorSTEMIn-situ TEM

A B S T R A C T

The piezoelectric charge coefficients d33 of single BaTiO3 (BT) nanoparticles (NPs) were characterized using atransmission electron microscope (TEM) that is equipped with a precise charge meter and an in-situ TEM in-dentation holder that enables controlled compression experiments. An exceptionally high d33 of 1775 pC/N wasobtained in NPs that are smaller than the critical diameter (D; typically known as<100 nm) that has beenregarded as the lower limit to permit for ferroelectricity in BT. The mechanical conversion efficiency of pie-zoelectric BT nanogenerators enhanced as D of BT NPs was decreased; this result corresponds with the single-NPcompression measurements of d33. This quantification of the effect of D in ferroelectric materials may guidedevelopment of efficient and high-powered nanostructured piezoelectric energy devices such as piezoelectricnanogenerators.

1. Introduction

Ferroelectric materials have a spontaneous internal electric polar-ization that can be reversed by an electric field [1]. Piezoelectricity isan ability to generate a voltage potential that is proportional to theapplied mechanical stress, or to generate a change in length in responseto applied voltage, and is one important property of ferroelectrics [2].This effect originates because mechanical deformation causes changesin dipole moment within non-centrosymmetric crystals, and generatesinternal polarizations [3,4]. Submicron biomedical electronics and ro-botics composed of piezoelectric devices such as nanogenerators, ac-tuators, sensors, and ultrasonic transducers are being developed, soinvestigations of the scaling effect in ferroelectric nanostructures re-quire identification of the particle/grain/crystallite size dependence ofphysical properties [5–7]. Non-centrosymmetric ferroelectrics showsize dependence of variation in the tetragonality [8]. The physicalproperties of nano-ferroelectric materials are different from those ofbulk states owing to the effects of size and surface area [9].

Piezoelectricity is induced by off-center atomic displacements froma delicate balance between Coulomb and covalent interactions [14]. Innanoscale ferroelectric crystals, both of these interactions can be

influenced by depolarization field, lack of periodicity, internal strain,and surface boundary; and these influences can significantly diminishthe intrinsic piezoelectric effect [15]. It is believed that this featurecould hinder the development of high-performance piezoelectric elec-tronic devices with nano-structured ferroelectric materials [16].

BaTiO3 (BT), which is one of well-known ferroelectric/piezoelectricmaterial, has been reported that the critical size (DC) of BT is about100 nm, and its tetragonality (c/a) can be reduced to ~1 at roomtemperature at diameter D< DC [2,10]. Furthermore, recent re-searches have shown that BT has a critical thickness of about 20 ~30 nm, and an individual BT nanocube with the size of 10 nm has apiezoelectric coefficient (d33) although this value was much smallerthan in bulk BT [11–13]. However, the origin of the piezoelectric BTparticles under 100 nm is not clear yet.

We report an exceptionally high d33 during deformation in singlepiezoelectric BT nanoparticles (NPs) that had D< DC. The experimentwas performed in a scanning transmission electron microscope (STEM)with an in-situ TEM indenter holder that enabled a controlled com-pression experiment. Direct electromechanical operations from BT NPsunder periodic compressive force as a function of particle size wereexamined to measure d33 and to infer the physical mechanism of this

https://doi.org/10.1016/j.nanoen.2018.12.096Received 30 September 2018; Received in revised form 30 December 2018; Accepted 31 December 2018

⁎ Corresponding authors.E-mail addresses: kwhokim@pusan.ac.kr (K.-H. Kim), jhryu@ynu.ac.kr (J. Ryu), youngchoi@postech.ac.kr (S.-Y. Choi).

1 Equally contributed authors.

Nano Energy 58 (2019) 78–84

Available online 08 January 20192211-2855/ © 2019 Elsevier Ltd. All rights reserved.

T

extraordinary piezoelectric response. As a practical demonstration ofpiezoelectric device, we developed piezoelectric nanogenerators thatuse BT NPs with various diameters, and compared the electric outputsof these devices.

2. Experimental section

2.1. Materials

The BT NPs, with diameters range of 10–1000 nm, were donatedfrom Samsung Electro-Mechanics Co. Ltd. For the electromechanicalcoupling measurement, the NPs were dispersed in ethanol by ultra-so-nication for 10min, and then the solution was dropped onto a 1 μm-wide Si plateau that was coated with a 10 nm-thick Au layer.

2.2. In situ TEM indentation

The measurements of charge generation from perovskite BT NPswere performed using a indenter holder in a TEM system (JEM-2100LaB6, JEOL) by repeated compression of NPs (Fig. 1a). The in-situ TEMindentation holder (PI 95 TEM PicoIndenter, Hysitron) contains anominally flat-ended (100) diamond indenter and a Si wedge substrate[17]. The diamond punch of the picoindenter can be manipulated withnanometer precision by a three-axis coarse positioner and a piezo-electric actuator to exert mechanical force on the BT particles. Periodiccompressive load from the diamond flat-punch indenter was appliedvertically to the BT NPs in a vacuum of 10−4 Pa. The compression wascontrolled by a closed-loop electrostatic force transducer. Duringcompression, the piezoelectric charges from BTs were measured using asensitive charge meter (Kistler 5015 A). This type of in situ method hasthe advantage of allowing high-resolution measurements of electriccharges, structural geometry, and contact forces, and thereby permitsquantitative and dynamic analysis of the nanoscale piezoelectric effect.The coincidently-aligned NPs with specific orientation (< 001> isparallel to the load applying direction) were selected to conduct theindentation experiment (Fig. 1b). The selected area diffraction pattern(SADP) of BT NP (Fig. 1b, inset) presents a sharply-confined spot pat-tern, which indicates that the BT particle was composed of high-quality(001) single crystals.

2.3. Atomic-resolution STEM for direct imaging of local polarization in BTNP

For direct observation of local polarization within BT NP, atomicallyresolved high-angle annular dark-field (HAADF) STEM images wereacquired at 200 kV accelerating voltage in a STEM (JEM-2100F, JEOL)equipped with a spherical aberration corrector (CESCOR, CEOS GmbH).The probe convergence angle of approximately 22 mrad was used. Theinner and outer angles of the HAADF detector were 70 and 200 mrad,respectively. The obtained HAADF STEM images were filtered using 2-dimensional Wiener filter to reduce background noise (HREM ResearchInc., Japan) [18]. The positions of each atomic column in the STEMimage were extracted by PPA (HREM Research Inc., Japan) [19]. Thestandard position of each unit cell was calculated from 4-neighboringBa atomic columns. From the obtained standard position, the positivecharge position was defined by Ti-atomic columns; the negative chargeposition was calculated by averaging the 4 oxygen atomic columns; andfinally polarization vectors from all unit cell were plotted from thenegative charge center to the positive charge center. We note that the Oatom positions are estimated under the assumption that both the O andthe Ti ions shift in the same direction. The atomic polarization (P) canbe simply calculated by the following Eq. (1), where Zi, δi, V is theeffective charge and displacement length of atom i, V is the unit cellvolume, respectively [20].

∑=PV

δ Z1

ii i

(1)

Polarization was calculated under assumption that displacement ofinvisible oxygen atoms along Ti-O atomic column behaves identical tothe other visible oxygen atoms in each unit cell. All of the atomic po-sition calculation and polarization vector plot were performed by thehand-made scripts based on MATLAB (Math Works).

2.4. Nanogenerator fabrication and measurement of output

To produce the piezoelectric nanogenerator, BT NPs with D =10,50, and 300 nm were blended with the fluidic polydimethylsiloxane(PDMS; Sylgard 184, Dow Corning, base and curing agent at ratio of10:1) [21,22]. The BT NPs/PMDS mixture fluid was sequentially coatedon the Al electrode (thickness of 200 nm) deposited polyimide film(thickness of 120 µm) by bar-coating process and cured at 70 °C for3min in an oven. To prevent formation of air bubbles in the

Fig. 1. Experimental set-up for measuring the piezoelectric charge from a BT NP. (a) Schematic diagram of the measurement system. Picoindentation TEM holderthat uses a diamond flat punch indenter and a Si wedge substrate, (b) TEM image of the indentation. Inset: Selected area diffraction pattern obtained from a BT NP.

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nanogenerators, degassing process was performed after mixing of BTNPs and PDMS as well as bar-coating process in a vacuum chamber.Then, this semi-cured piezoelectric nanocomposite was attached toanother Al-coated polyimide film. The Al layers had a role of chargecollector in the nanogenerator. After fully hardening the nanocompo-site device inside 60 °C oven for 1 day, two Cu wires were connected tothe top and bottom Al electrodes by conductive silver epoxy (CW2400,Chemtronics). The piezoelectric nanogenerator was poled by an electricfield of 1 kV/mm at 70 °C for 3 days. For characterizing output currentof the BT nanogenerator, a custom designed pushing stage was utilizedto apply a periodic vertical force of 100 N, and the generated short-circuit current signals were recorded by a multi-meter (Keithley2611 A) [23].

3. Results and discussion

3.1. Measurement of piezoelectric signals from the repeated indentation ofBT NP

A square-wave load function was applied to a BT NP that had D=80 nm, and the charge was measured. The amplitude of the pushingforce (ΔF= Fcompressed - Frelaxed) was set to 50 μN, and the generated

electric charges were measured during several cycles (Fig. 2a). Theelectromechanical responses were most obvious when the force wasapplied or released; this result means that the electromechanicalcharacteristic depends on the strain rate, rather than as on the strainitself. The polarity of the measured electric charge was negative duringapplication of strain, and positive during release. This result is clearevidence that the signal from the BT NPs is caused by the piezoelectriceffect [24]. The standard deviation of the electric charge from repeatedcompression and relaxation of BT NP was within 7% of the mean value;this result means that a stable mechanical contact was preservedthroughout all loading cycles. In the compressed state (Fig. 2b),bending contours were generated; their directions were normal to theforce applied direction. After the indentation (relaxed state), lineardefects and elastic deformation disappeared at the surface of the NP(Fig. 2c).

3.2. Size dependent piezoelectric charge constant of BT NPs

The force-dependent electromechanical charge responses from thevarious BT particles were examined by a series of indentation experi-ments on a BT NPs. Data points were calculated by averaging the heightof the peaks (n > 10 per condition) in the compressed state from each

Fig. 2. Piezoelectric signal and TEM images acquired during the repeated indentation of a BT NP . (a) Square-wave load function applied to the particle (Upperpanel), and electric charge measured during repeated compression and release (Lower panel). (b), (c) TEM images of the particle in compressed state (51 μN) (b) andrelaxed state(c).

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indentation set utilized in Fig. 2a. The electric charges emitted from the50–1000 nm sized NPs increased linearly as ΔF increased (Fig. 3a). Thed33 quantifies the polarization change (electric charge separation in thematerials) when a ferroelectric material is subjected to a mechanicalstress (units of C/N) or the length change on application of an electricfield (units of m/V). We could measure d33 of the single BT NP by de-termining the slope of the linear regression line in Fig. 3a. The size-dependent piezoelectric response from the BT NPs was quantified byconducting a series of compressions on BT NPs with D =50, 80, 120,300 or 1000 nm (BT measurements using STEM cannot be performedfor D<50 nm). The d33 increased non-linearly as the D decreased. BTNPs with D =300 and 1000 nm had d33 ~270 pC/N, which is com-parable to the bulk property of a BT single crystal (~300 pC/N),whereas BT NPs with D ≤ 120 nm had d33> 1500 pC/N, which is anorder of magnitude greater than d33 in the bulk state (Figs. S1 and S2)[25]. Especially, 50 nm BT NP had d33 = 1775 pC/N, which is ~ 6times larger than typical value of a BT bulk single crystal (Fig. 3b).

The size effect on nanoscale ferroelectricity in the classic perovskite-structured ferroelectric BT has been a matter of debate. The inversepiezoelectric effect on a 10 nm ferroelectric BT nanousing piezo-response force microscopy (PFM) on a 10-nm ferroelectric BT nanocubedetected d33 = 1.55 pm/V, which is severely reduced from its bulkvalue [12]. However, in our case, the direct piezoelectric effect alloweddetection of extremely high d33 of BT NPs at D< 120 nm. The reasonfor this difference in piezoelectric coefficients obtained using reverseand direct piezoelectric responses in extremely small BT NPs might bethe structural flexibility. The larger Ti-displacement, ~0.14 A was re-ported via synchrotron XRD analysis even by diminishing the tetra-gonality in BT NP [26], while the Ti atoms in the bulk BT are ~ 0.05 Aoff-centered along [001] from the cubic center made of Ba atoms, whichresults from Ti-O hybridization in the low symmetric structure [27].Three times larger Ti-displacement in BT NPs implies that BT NP can bepolarized with the structural flexibility. The atomic-resolution STEManalysis was performed to verify the phenomenological basis of theabnormal increase in d33 in NPs with D< 120 nm (Fig. 4 and S4). Thisexceptionally huge piezoelectric response may be a result of structuralflexibility of nanoparticles [28]. The strain-stress curve (Fig. S3a)showed that the 80 nm BT particle can tolerate and recover from a hugestrain of up to 20%; and 100 nm and 120 nm particles can be strainedup to ~10%. By referring that Al2O3 shows the maximal strain of

approximately 0.6% before its fracture, BT NPs with D< 120 nm ex-hibit the extraordinary structural flexibility. More interestingly, Young'smodulus of 80 nm particle is 47 GPa, which is much lower than thereference value, ~120 GPa [29]. Therefore, this structural flexibilitycan give rise to the excellent piezo-response, d33> 1500 pC/N.

3.3. Atomic resolution STEM characterizing of BT NT for verification oflocal polarization and tetragonality

A spontaneous distortion of the crystal lattice of piezoelectric BTarises from a relative displacement along the c-axis of the Ti cation withrespect to its centrosymmetric position. To confirm the nanoscale dis-placement, we observed the (100)-orientation BT NP with 10 nm dia-meter by using HAADF imaging using a STEM with a spherical aber-ration corrector. From atomic-scale HAADF STEM image (Fig. 4a),ferroelectric polarization map (Fig. 4b), and c/a ratio (Fig. 4c) for a BTNP was obtained. All elements including Ba and Ti atoms can be clearlyclassified in the HAADF-STEM image, so the voluntary polarization isconfirmed. The view of local Ti displacements at the level of unit cellwas obtained by calculating relative displacements between O-Ti col-umns and Ba sublattices in each unit cell after reinterpretation of phaseimage by using a method described earlier [30,31]. The image-pro-cessed map presents alignment of the Ti displacements for a unit latticethrough a [100] projection (Fig. 4b), but this map does not correspondto a tetragonality denoted by c/a ratio (Fig. 4c). The polarization of theBT NP seems to be rather randomly oriented; this observation contra-dicts the commonly-held view that tetragonal structure gives rise todominant ferroelectricity with a linearly-aligned polarization. In mag-nified regions (Red (i) and yellow (ii) square in Fig. 4b), Ti displace-ment was linear through several unit cells, so a polarization of up to ~100 μC/cm2 can be expected. This local, intense polarization of ~ 100μC/cm2 is much higher than the normal polarization of ~ 26 μC/cm2

from the bulk BT [32]. However, in these regions the c/a ratio is veryclose to 1, which means that the polarization in the nanoparticles couldbe existed on the unit cells despite its weak lattice tetragonality.Therefore, the ferroelectricity in BT NPs does not disappear with ap-pearance of paraelectricity, but it is depolarized by the random or-ientation, which could be a feasible approach due to its structuralflexibility. This observation implies that the ferroelectricity in BT NPscan also occur by structural modulation or alignment of the electric

Fig. 3. Linear relation of applied compressive force vs generated piezoelectric charge from BT NPs and size-dependent piezoelectric coefficient. (a) Applied com-pressive force vs measured electric charge graph. The measured electric charge values from the various BT NPs in the range of D=50–1000 nm were calculated byaveraging the peak values at the compressed state. (b) Piezoelectric coefficient from nanoparticles with different sizes. Each value is the slope of a linear fit of force vsmeasured electric charge graph in each size of nanoparticle.

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field. Other NPs that showed a similar tendency were also observed(Fig. S4).

3.4. Piezoelectric output from nanogenerators with different-sized BT NPs

To determine whether this size-scaling effect of BT NPs can beexploited in practical electronic devices, we fabricated composite-structured piezoelectric nanogenerators that included various-sized BTparticles as active electromechanical conversion elements [33]. BT NPswith D =10, 50, and 300 nm were dispersed in PDMS elastomericmatrix to construct a nanocomposite that had dimensions of20×20×3mm3. The BT composite layer was sandwiched betweendifferent top and bottom Al-coated polyimide films (Fig. 5a). The BTpiezoelectric nanogenerators were induced to generate electricity bysubjecting them to a periodic vertical force by using an automaticpushing machine (Fig. 5b). This repeated deformation generated cur-rent due to the piezoelectric effect inside the BT nanocomposite layer.The short-circuit output current signals of piezoelectric nanogeneratorsincreased with as D was decreased: 10, 50, and 300 nm BT NPs gen-erated maximum currents of 32, 32, and 4 nA (Fig. 5c). These resultsagree with the piezoelectric responses obtained by compression(Fig. 3b). The generated total charge Q (same as output current ofpiezoelectric nanogenerator) of piezoelectric material at a short-circuit

state can be simply described by Q=A·ε·E·d, where A is the surface areaof the piezoelectric phase, ε is the strain, E is the Young's modulus, andd is the piezoelectric charge coefficient [34]. A large piezoelectric d33 of10 and 50 nm BT NPs could strongly affect the increase in currentoutput of nanogenerators compared to relatively large NPs with D=300 nm. This result shows that inhomogeneous mechanical strain inthe smaller NPs may still be generated using an external force even inthe elastomeric matrix, and that the piezoelectric effect of infinitesimalferroelectric nanostructure can be successfully utilized to develop ahigh-efficiency piezoelectric nanogenerator.

4. Conclusions

To summarize, we report a remarkable piezoelectric constant d33= 1775 pC/N from BT NPs with D< 120 nm, by compressing singleparticles on a TEM stage. The amount of generated electric charge fromthe direct electromechanical response of individual BT NP was pro-portional to the applied compressive force on the piezoelectric material.The measured d33 increased rapidly as D was decreased; NPs with D=300 nm and 1000 nm had d33 ~260 pC/N, whereas NPs with D=50,80, and 120 nm had> 1500 pC/N. This result contrasts with previousreports that show a huge decline of d33 with decrease in D of BT NPs.The TEM-based mechanical analysis shows that a small NP had a higher

Fig. 4. Atomic resolution STEM measurements of 10 nm BT NP for verification of local polarization and tetragonality. (a) Filtered HAADF STEM image of 10 nmBaTiO3 nanoparticle, showing Ba and Ti atomic positions. The enlarged images (i) and (ii) are examples of strongly, and randomly distorted regions of Ti atom in eachunit cell. (b) Polarization map estimated from atomic displacements. Arrows denote the polarization direction. (c) Tetragonality c/a map obtained from Ba atomicpositions shown in (a).

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deformation rate than a large NP; this difference may be the cause ofthe high d33 of BT NPs with D< 120 nm. A piezoelectric nanogeneratorthat used BT NPs with 10≤D ≤ 300 nm generated current that in-creased as D decreased. This result concurs with the single-NP in-dentation measurements of d33. This work suggests an explanation forthe high piezoelectric conversion efficiency on minuscule BT NPs andextends the application of nanoscale ferroelectrics to real high-outputpiezoelectric devices.

Acknowledgements

This research work was mainly supported by the Global Frontier R&D Program on Center for Hybrid Interface Materials (HIM) funded bythe Ministry of Science, ICT & Future Planning Korea (Grant no. NRF-2013M3A6B1078872 & 2016M3A6B1925390); the National ResearchCouncil of Science & Technology (NST) grant by the Korea government(MSIP) (No. CAP-17-04-KRISS).

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2018.12.096.

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Fig. 5. Piezoelectric responses (electric current) from nanogenerator devices with different-sized BT NPs. (a) Schematic of piezoelectric nanogenerator including BTNPs. (b) Optical image of nanogenerator device with the pushing state. (c) The measured short-circuit current signals from the nanogenerators including various-sized BT particles PT during periodic pushing motions.

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Dr. Sung-Dae Kim is senior researcher in Korea Institute ofMaterials Science (KIMS), South Korea. He received his PhDin materials science and engineering from Seoul NationalUniversity, South Korea in 2013. His current research in-terests are in microstructure characterization using electronmicroscopy.

Dr. Geon-Tae Hwang is a senior researcher at KoreaInstitute of Materials Science (KIMS), Korea. He receivedthe Ph.D degree from materials science and engineering atKorea Advanced Institute of Science and Technology(KAIST). He has studied for piezoelectric energy harvestersand magnetoelectric composites.

Dr. Kyung Song received her Ph. D. in the Department ofMaterials Science and Engineering at Pohang University ofScience and Technology (POSTECH), Korea in 2015 andcurrently she is a senior researcher at Korea Institute ofMaterials Science (KIMS). Her research interests focus onunderstanding of atomic and electronic structure of newfunctional materials and developing the real-time electric/magnetic field imaging with electron microscopy.

Prof. Chang Kyu Jeong is a professor in the Major ofElectronic Materials Engineering within the Division ofAdvanced Materials Engineering at Chonbuk NationalUniversity. He received his B.S. degree from HanyangUniversity and his M.S. and Ph.D. degrees from KoreaAdvanced Institute of Science and Technology (KAIST),respectively, in Materials Science and Engineering. Afterworking on a postdoctoral research fellow in Institute forNanoCentury (KINC), he was employed as a postdoctoralscholar in Pennsylvania State University. His current re-search topics are ferroelectric materials, biological andecological electromechanical properties and their devicesfor energy and sensor applications in future soft logics.

Prof. Kwi-Il Park is a professor in the Department ofMaterials Science and Metallurgical Engineering atKyungpook National University. Prof. Park received hisPh.D. in Materials Science and Engineering at KoreaAdvanced Institute of Science and Technology (KAIST). Hewas previously a senior researcher at Korea Agency forDefense Development from 2014 to 2015 and a facultymember at Gyeongnam National University of Science andTechnology from 2015 to 2018. His research interests in-clude the synthesis of high-performance piezoelectric na-nomaterials and development of flexible/stretchable energyharvester based on inorganic piezo-materials.

Mr. Jinhyuk Jang is a graduate school student in theDepartment of Materials Science and Engineering at PohangUniversity of Science and Technology (POSTECH) since2017. His research topic is to understand the physicalproperties of the functional oxides on an atomic scale byusing aberration-corrected electron microscopy.

Prof. Kwangho Kim received his PhD’s degree from KoreanAdvanced Institute of Science and Technology (KAIST) in1986 and is a professor in Division of Materials and ScienceEngineering of Pusan National University since 1985. He isDirector of National Core Research Center for HybridMaterials Solution, Director of National ResearchLaboratory, vice president of the Korean Ceramic Society,Board of Trustee of National Research Foundation of Korea,Editorial Board of Korean Union of Chemical Science andTechnology Society, etc. His researches are focused to hy-brid-interface materials for future innovations includingdesign-syntheses-evaluation of nano-hybrid materials andhybrid-functional hard coatings

Prof. Jungho Ryu received his Ph. D. in materials scienceand engineering at Seoul National University, Korea in2001. During his Ph.D and later in his post-doctoral tenure(2001–2003), he had studied magnetoelectric compositesand piezoelectric materials at the Pennsylvania StateUniversity, USA. Prior to joining Yeungnam Univerity in2018, he was a senior engineer and project leader atSamsung Electro-Mechanics Co. Ltd, Korea (2003–2006)and a principal researcher at Korea Institute of MaterialsScience (KIMS), Korea (2006–2018). His current researchinterests include the development of piezoelectric and fer-roelectric devices, energy harvesting, and functional thin/thick ceramic films.

Prof. Si-Young Choi is an associate professor in theDepartment of Materials Science and Engineering at PohangUniversity of Science and Technology (POSTECH). He hadreceived his Ph. D. degree at Korea Advanced Institute ofScience and Technology (KAIST) in 2004. Since then, hewas a post-doctoral researcher in the field of electron mi-croscopy at Oxford University and a JSPS fellow researcherat the University of Tokyo. He was a principal researcher inKorea Institute of Materials Science (KIMS, 2008–2017).His specialty is the atomic scale analysis via aberration-corrected STEM in the variety of functional materials, suchas ferroelectric/piezoelectric perovskite oxides, Li-ion bat-tery cathode oxides, multiferroic oxides, and so on.

S.-D. Kim et al. Nano Energy 58 (2019) 78–84

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