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RAPID COMMUNICATION

Depletion width engineering via surfacemodification for high performancesemiconducting piezoelectric nanogenerators

Keun Young Leea,1, Jihyun Baeb,1, SeongMin Kimb,1,Ju-Hyuck Leec, Gyu Cheol Yoona, Manoj Kumar Guptaa,Sungjin Kima, Hyeok Kimb, Jongjin Parkb,n, Sang-Woo Kima,c,nn

aSchool of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 440-746,Republic of KoreabSamsung Advanced Institute of Technology, Yongin 446-712, Republic of KoreacSKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT),Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea

Received 15 March 2014; received in revised form 15 May 2014; accepted 3 June 2014Available online 13 June 2014

KEYWORDSPiezoelectricnanogenerator;Zinc oxide;Poly(3-hexylthio-phene);Surface modification;Depletion widthengineering

0.1016/j.nanoen.2lsevier Ltd. All rig

thor.uthor at: School oterface Nanotech90 7381.: jongjin00.park@ntributed equally

AbstractPiezoelectric semiconductor materials have emerged as the most attractive material fornanogenerator (NG)-based prototype applications, such as piezotronics, piezophotonics andenergy harvesting, due to the coupling of piezoelectric and semiconducting dual properties.Understanding the mechanisms for high power generation, charge transport behavior, energyband modulations, and role of depletion width in piezoelectric semiconducting p–n junction,through piezoelectric charges developed by external mechanical strains, are essential forvarious NGs. Here, we demonstrate enhancement of the output power of one-dimensional zincoxide (ZnO) nanowires (NWs)-based NG using a p-type semiconductor polymer, by controllingtheir energy band at depletion width in the piezoelectric semiconducting p–n junction interfaceand native defects presented in as-grown ZnO NWs. The piezoelectric output performance fromthe P3HT-coated ZnO NWs-based NG was several times higher than that from the pristine ZnONWs-based NG, under application of the same vertical compressive strain. Holes from thep-type P3HT polymer significantly reduced the piezoelectric potential screening effect causedby free electrons in ZnO. Theoretical investigations using COMSOL multiphysics software were

014.06.008hts reserved.

f Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology (SAINT),nology (HINT), Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea. Tel.: +82 31 290

samsung.com (J. Park), kimsw1@skku.edu (S.-W. Kim).to this work.

K.Y. Lee et al.166

also carried out, in order to understand the improvement in the performance of surfacepassivated ZnO NWs-based NG, in terms of free carriers concentration and holes diffusion, dueto the formation of p–n junction at the interface of ZnO and P3HT, and depletion width change.& 2014 Elsevier Ltd. All rights reserved.

Introduction

Piezoelectric nanogenerators (NGs) are one promising wayto scavenge the energy from various sources, includingmechanical vibration, ultrasonic wave, air/liquid pressures,and even sound waves [1–4]. Semiconducting piezoelectricmaterials show great advantage over insulating piezoelec-tric materials in many fields, such as piezoelectric diodes,piezoelectric field effect transistors, piezophotonics, andpiezophototronics application, due to their attractive coex-isting dual properties [5–7]. Semiconducting piezoelectricmaterials such as ZnO have attracted a great deal ofattention for use in high performance piezoelectric NGs,but the piezoelectric potential screening effect induced bythe presence of the free carrier density in piezoelectricsemiconducting materials is one of the most important andfundamental issues that prevents the realization of stableand large power output performance NGs. Further, utilizingthe coupling of piezoelectric and semi-conducting dualproperties of ZnO, many potential applications have beensuccessfully demonstrated, such as tuning the performanceof photo cells, photo detectors, and light-emitting diodebased on p–n junction by the piezoelectric effect [8–14].Although modulations of charge depletion width and energyband bending at the p–n junction interface and metal–semiconductors interface via strain induced piezoelectriccharges, and their effect on output performance in semicon-ducting piezoelectric material are interesting and attractivephenomena, only a few studies have been reported.

In spite of this, ZnO can generate an intrinsic piezo-electric potential of a few volts, as a result of its mechan-ical deformation; but free carriers exist in ZnO, whichusually screen some part of the piezoelectric potential thatis generated. In addition to the free electrons in ZnO,several monolayers of hydroxide (OH) at the surface ZnO aremostly present due to H adsorption, which introduces anelectron accumulation layer at the surface, which increasethe conductivity of ZnO, which further leads to reduction ofthe piezoelectric potential [15]. Therefore, improving theperformance of ZnO-based NGs is a considerable under-taking, and several methods, such as surface passivation,plasma treatment, and thermal annealing, have beenemployed by many researches [16–24]. However, the pre-sence of the OH groups at the surface of ZnO will bedesorbed, after annealing at temperatures of 150 1C, andthe effect of plasma treatment on ZnO is likely not verystable under atmospheric conditions. Therefore, a betterenvironment to isolate the ZnO-based NGs from the atmo-sphere is essential. Recently, our group also reported theenhancement of piezoelectric power output from ZnO thinfilm-based NGs, by introducing a p-type polymer layer poly(3-hexylthiophene) (P3HT) on ZnO thin film [16]. We pro-posed that holes at the film surface of P3HT greatly reduce

the piezoelectric potential screening effect caused by freeelectrons in a piezoelectric semiconducting materials,which results in high output power. However, improvingthe output performance of NGs, through incorporating P3HTwith one-dimensional ZnO, such as nanowire (NW) nanorods,has not yet been reported. One-dimensional ZnO nanos-tructures have been considered as promising candidates forapplications such as energy harvesting, nanosensors andmany nano-scale devices, owing to their high surface-to-volume ratio. Therefore, theoretical understanding of thesurface passivation in ZnO NWs, formation of interfacecharge-depletion region, and their effects on piezoelectricpotential under mechanical strain is urgently required, toengineer the design of high performance piezoelectricenergy harvesters, for powering nano-scale electronicsdevices and self-powered nanosystems.

In this paper, we report the enhancement of outputpower from ZnO NWs-based NGs, by hybridizing p-typesemiconducting P3HT polymer. We controlled the nativedefects in ZnO NWs, and modulated the energy bandalignment in self-formed p–n junction, for high poweroutput. We measured the output voltage and current fromboth vertically aligned pristine ZnO NWs-based NGs andP3HT-coated ZnO NWs-based NGs, under vertical compressivestrains. The output results show the P3HT can significantlyincrease the output voltage and current, as compared topristine ZnO NWs-based NGs, due to surface passivation andthe formation of p–n junction. We report that a shift in theFermi level helps in increasing the power output of ZnO NWs-based NGs. We also proposed a theoretical model for investi-gating the dependence of the piezoelectric potential of ZnONWs on free carrier concentrations, surface defects and nativedefects, and their possible passivation by P3HT polymer. Thetheoretical calculation and experimental data were found to bein close agreement.

Experimental

Fabrication of flexible NGs

ZnO NWs were synthesized on indium tin oxide (ITO)-coatedpoly(ethylene 2,6-naphthalate) (PEN) substrate, using theaqueous solution method. Zinc acetate dehydrate [Zn(CH3COO)2 � 2H2O, 0.03 M] dissolved in ethanol (100 mL)was prepared as a seed solution. The seed solution wasthen spin-coated on ITO/PEN substrate at 1000 rpm for 60 s.After 6 times of spin-coating and annealing, the spin-coatedsubstrate covered with a ZnO seed layer was dried onto ahot template at 95 1C. Vertically aligned ZnO NWs wereformed on the ITO/PEN substrate, by immersion into anaqueous solution consisting of zinc nitrate hexahydrate [Zn(NO3)2 � 6H2O, 0.05 M], hexamethylenetetramine (0.05 M),

167Depletion width engineering via surface modification

and de-ionized water (400 mL). The main growth of the ZnONWs was undertaken at 95 1C for 5 h. A 100-nm-thick gold(Au) layer was deposited on another PEN substrate, using athermal evaporator. The pristine NG was formed by inte-grating the ZnO NWs/ITO/PEN with the Au-coated PEN as atop electrode. For fabricating the P3HT-ZnO NWs-based NG,p-type semiconducting P3HT was synthesized on the top ofn-type ZnO NWs, which subsequently form a heterojunctionwith the interface of n-type ZnO NWs, and then Au-coatedPEN electrode was placed, for Schottky contact formationbetween the Au and P3HT-coated ZnO NWs.

Characterization and measurements

Field-emission scanning electron microscopy (FE-SEM) andX-ray diffraction (XRD) measurements were performed, forthe morphological and structural investigation of ZnO NWs.

Figure 1 Schematic diagrams of (a) vertically aligned pristine ZnOZnO NWs-based flexible NG on ITO-coated plastic substrate. (c) PlanZnO NWs. (e) Cross-sectional FE-SEM images of pristine ZnO NWs an

A pushing tester (Labworks Inc., model no. ET-126-4) wasused, to create strain in the NG. A Tektronix DPO 3052Digital Phosphor Oscilloscope and low-noise current pream-plifier (Model No.: SR570, Stanford Research Systems, Inc.)were used for the electrical measurements.

Results and discussion

Figure 1a and b shows schematic diagrams of integrated NGswith pristine piezoelectric ZnO NWs and P3HT-coated ZnONWs, respectively. Figure 1a shows that vertically alignedZnO NWs was grown on ITO-coated PEN substrate, and theAu-coated PEN electrode was placed above the ZnO NWsarrays, for Schottky contact formation between the Au andZnO NWs. Figure 1c shows an FE-SEM image of the verticallywell-aligned hexagonal ZnO NWs, on the ITO-coated PENsubstrate. The average diameter and length of the ZnO NWs

NWs-based flexible NG and (b) P3HT-coated vertically aligned-view FE-SEM images of pristine ZnO NWs and (d) P3HT-coatedd (f) P3HT-coated ZnO NWs.

K.Y. Lee et al.168

were 100 nm and 1 mm, respectively. FE-SEM analysis of theP3HT-coated ZnO NWs sample was carried out, to gaininsight on the nanoscale features within the structure, asshown in Figure 1d. The P3HT film thickness was approxi-mately 30710 nm width and 1 mm length of the ZnO NWs.Cross-sectional images of vertical aligned pristine ZnO NWsand P3HT-coated ZnO NWs are shown in Figure 1e and f,respectively. Further, XRD was performed to investigate thecrystal structure of grown ZnO NWs. XRD studies confirmedthe formation of a hexagonal system of ZnO NWs (Figure 2).All the diffraction peaks coincide with the earlier reportedvalues, with lattice parameters a=3.249 Å and c=5.206 Å

Figure 2 XRD spectrum of vertically aligned pristine ZnO NWson ITO-coated PEN substrate.

Figure 3 Experimentally observed (a) output voltage and (b) cuvoltage and (d) current from P3HT-coated ZnO NWs-based NG, und

(JCPDS Card no. 36-1451). It can be seen that vertical ZnONWs show a sharp XRD reflection peak detected at �34.41,suggesting a preferential growth along the c-axis normal tothe flexible ITO substrate.

We measured the output voltage and current generatedby the both the NGs, by applying a pushing force (0.8 kgf)perpendicular to the NGs, using a mechanical force stimu-lator, which has an effective size of 1 cm� 1 cm (Figure 3aand b). We observed an alternating current (AC)-type out-put voltage of about 30 mV, with a small AC-type currentdensity (200 nA/cm2), from a pristine ZnO NWs-based NG,while P3HT hybridized ZnO NWs-based NG showed very highoutput voltage and current density, with average value upto 300 mV and 1 mA/cm2, respectively, under the samevertical compressive strain (Figure 3c and d). This enhance-ment in the output voltage is due to the increase in thepiezoelectric potential, via neutralization (passivation) ofthe free electrons existing in the ZnO layer, by attractingholes from the P3HT and p–n junction formation. Since atthe ZnO–P3HT interface, holes near the interface from theP3HT tend to diffuse into the ZnO layer, and free electronsfrom the ZnO begin to diffuse into the P3HT, the combina-tion of electrons and holes forms an interface charge-depletion region, and consequently a p–n junction. In detail,we can understand the power generation and enhancementmechanism, as shown in Figure 5. Polarity-switching testswere also carried out to confirm that the output per-formance originated from the piezoelectric phenomenon.

rrent from pristine-based ZnO NWs-based NG, and (c) outputer vertical compressive force of 0.8 kgf.

169Depletion width engineering via surface modification

An opposite output signal is observed when the device isconnected in reverse connection as shown in Figure 4.

When the pristine ZnO NWs-based NG and P3HT-coatedZnO NWs-based NG are subjected to a vertical compressivestrain, a negative piezoelectric potential is set up at the topside of the ZnO NWs, and a positive piezoelectric potentialis set up at the bottom surface of the ZnO NWs, which drive

Figure 4 The polarity-switching tests (forward and reverseconnections) demonstrate that the output signals are fromP3HT-coated ZnO NWs-based NG rather than the instruments.

Figure 5 Proposed mechanism for piezoelectric charge gene(a) piezoelectric negative and positive charges are induced acrosspushing force. The piezoelectric potential induced electrons areinterface between the electrode and the side of nanorods with posfree electrons of ZnO NWs, which result in the low observed piezoelflow from the bottom electrodes to the top electrode, resulting inP3HT-coated ZnO NWs. (f) Due to the p-type P3HT coating, holestherefore the screening effect is significantly reduced, which resul(g) When the external force is removed, the piezoelectric potentelectrons flow back, via the external circuit.

the piezoelectric potential-induced electron flow from theAu electrode to the ITO electrode through an external loadresistor, giving rise to a positive voltage and current pulseunder forward connection (Figure 5a and b). When thestrain is released, the piezoelectric potential immediatelyvanishes, and the electrons accumulated near the ITOelectrode flow back through the external circuit to the Auelectrode, giving rise to a negative pulse, and returning thesystem to its original state. However, in the case ofuncoated ZnO NW based NG, the output voltage and currentwere very low, due to the screening effect, due to freecarriers and surface defects of ZnO NWs (Figure 5b and c).

On contrary, in the case of P3HT-coated ZnO NWs-basedNG, while the main mechanism remains same, the poweroutput was very high, because at the ZnO–P3HT interface,holes near the interface from the P3HT tend to diffuse intothe ZnO layer, and free electrons from the ZnO begin todiffuse into the P3HT; the combination of electrons andholes forms an interface charge-depletion region, andconsequently, a p–n junction. Therefore, the enhancementin the output voltage is due to the increase in the piezo-electric potential, via neutralization (passivation) of thefree electrons existing in the ZnO NWs, by attracting holes

ration and output power enhancement mechanism of NGthe top and bottom of ZnO NWs, respectively, under verticalthen moved via the external circuit, and accumulate at theitive potential. (b) A few piezoelectric charges are screened byectric output. (c) During releasing state, accumulated electronsa negative pulse. (e) Piezoelectric charges created inside theare diffused, and cancel the free electrons of ZnO NWs, andts in the high output that was obtained across the electrodes.ial inside the NWs instantly disappears, and the accumulated

2 þ

K.Y. Lee et al.170

from the P3HT and p–n junction formation (Figure 5e).Further, the ZnO NWs surfaces are very rich in defects,predominantly oxygen vacancies, which are electrondonors, and also serve as the binding sites for chemicaladsorption processes; therefore, surface passivation due toP3HT polymer can eliminate these defects, and also makethe surface more chemically inert, i.e. more resistive to Hatom adsorption. In addition, the potential generatedacross the ZnO layer is also sufficient to cause bandmodulation that leads to band bending at the ZnO–P3HTand Au–ZnO junction, which modulates the Schottky barrier,and therefore outputs piezoelectric signals. Moreover, addi-tional carriers from a conducting polymer and shift in theFermi level help in increasing the power output.

In addition, the effective dielectric constant of the ZnONW coated with P3HT polymer decreases as compared tothe pristine ZnO NW due to a lower dielectric constant ofP3HT (�3) than that of ZnO (�8). Therefore, it can besuggested that the higher power output obtained from theP3HT-coated ZnO NW-based NG may also be attributed tothe dielectric constant lowering. The output voltage gen-erated from the ZnO NWs-based NGs is described asVout=g33εE, where g33 is the piezoelectric voltage constant,ε is the strain, and E is the Young's modulus. The piezo-electric voltage constant, g33, is proportional to the piezo-electric charge coefficient (d33) [g33=dλ/(ε0K), where ε0 isthe permittivity of free space and K is the relative dielectricconstant of the material], indicating the increase of Vout bythe dielectric constant lowering [25,26].

Further, numerical simulation of piezoelectric semi-conductor-based p–n junction is done using the COMSOLmultiphysics software package, to understand the chargetransport behavior and energy band modifications at theinterface. In simulation, we conveniently manipulate ap-type ZnO (only material name in the software) on top ofan n-type ZnO NW, instead of a p-type semiconductingpolymer of P3HT, with the same donor (Nd) and acceptor(Na) concentration of 1017 cm�3. The p-type is assumedhere to be non-piezoelectric, due to the characteristicproperty of P3HT polymer and the well-known dielectricconstant of P3HT is used. The reason for modeling such aconfiguration is that the actual piezoelectric generationoccurs locally around the tip of an n-type ZnO NW, due tothe vertical compression that is induced by the force in thedevice. The length and diameter of NW used in our modelparameter were 1 μm (i.e. 500 nm for n-type, and 500 nmfor p-type region) and 200 nm, respectively.

Through theoretical simulation of piezoelectric p–n junction,piezoelectric equations, the convection and diffusion equations,and continuity equations are coupled and solved. For the dopantconcentration function, N can be approximated, as below.

N¼ Nþd ðfor n�type regionÞ; N¼ N�

a ðfor p�type regionÞð1Þ

where Nd+ is the ionized donor concentration and Na

� is theionized acceptor concentration. N is assigned to have anegative value in the p-type region and positive value in then-type region. Further, the electrostatic behavior of chargesin a piezoelectric ZnO NW is described by

εs∇2ψ i ¼ �qðp�nþNþρδÞ ð2Þ

εs∇ ψ i ¼ �qðp�nþNd þρpiezoÞ for n type region ðZnOÞð3Þ

εs∇2ψ i ¼ �qðp�n�N�a Þ for p�type region ðP3HTÞ ð4Þ

where ψi, εs, ρ, δ and q are the electric potential distribu-tion; permittivity of the materials used; charge densitydistribution (subscript piezo means charge density inducedfrom the piezoelectricity of ZnO by mechanical deforma-tion); delta function, which only becomes 1 when thecharge density is induced from piezoelectricity (ρ=ρpiezo),and if not, becomes 0; and q is the absolute value of unitelectronic charge, respectively. n and p are the concentra-tions of free electrons and free holes, respectively.

Further, for the electron–hole recombination in a p–njunction, three recombinations are used, including the bandto band recombination, Auger recombination, and trap-assisted recombination (Shockley–Read–Hall recombination)[27]. However, there is no external optical excitation; thuselectron and hole generation rates are neglected for thepresent case (no light signal was applied on the device).Therefore, the recombination rates of electron and hole forband-to-band (ubb), Auger (uAuger) and Shockley–Read–Hallprocess can be expressed as

ubb ¼ bðnp�n2i Þ ð5Þ

uAuger ¼ ðnp�n2i ÞðΓnnþΓppÞ ð6Þ

uSRH ¼ np�n2iτpðnþniÞþτnðpþniÞ

ð7Þ

where ni is the intrinsic carrier density of 1� 106 cm�3, τpand τn are hole and electron life times of 0.1 μs, respec-tively, b is the bimolecular recombination constant of1.5–1.7� 10�10 cm3/s [28], and Γn and Γp are the Augercoefficients for e�/h+ of 1.14� 10�31 cm6/s [29]. Notethat the electrical boundary condition at one end of the p–njunction is assumed to be ground, and the carrier concen-tration at the electrode is given by Ref. 27.

Figure 6a shows the calculated axial electrical potentialalong the z-axis, with various vertical stress of Tz (from0 to �1� 108 Pa). When the Tz=0 (i.e. force is 0), theelectrical potential drops down to �0.7 V, which is theso-called built-in potential (Vbi). As the force increases byTz=�1� 107 to �1� 108 Pa (minus sign means that thedirection is �z), the potential drop in the n-type regionalso increases with the depletion width. The variantdepletion width, particularly in the p-type region, isexplained on the basis of the generated negative piezo-electric charges at the top surface of the n-type ZnO(at the interface between the n-type and p-type semi-conductor), due to the applied compressive strain thatresults, as induced negative piezoelectric charge repelsthe free carriers (electrons) of ZnO towards the end ofn-type semiconductor. In the meanwhile, holes in thep-type region accumulate, due to negative piezoelectriccharges, which process then leads to relatively narrowdepletion width in the p-type region.

Figure 6b shows a comparison of the calculated piezo-electric potential between the p–n junction NW and n-typeNW as a reference. To calculate the exact potential of a p–n

Figure 6 COMSOL simulation results. (a) Variation of electricalpotential of p–n junction NW along the z-coordinate at differentapplied stresses, (b) comparison of output piezoelectric poten-tial from p–n junction NW with n-type NW, and (c) variation ofelectron concentration at various applied stresses along thez-coordinate.

171Depletion width engineering via surface modification

junction NW generated by only the piezoelectric effect, thebuilt-in potential by the p–n junction at Tz=0 has beensubtracted from the total electrical potential. The obtainedresult shows 1.5–2 times enhancement of piezoelectricpotential for the case of the p–n junction NW, comparedto the n-type NW. Figure 6c shows the electron concentra-tion profile of the p–n junction NW along the z-axis.A depletion region is also observed at the p–n junctioninterface, and it becomes wider, as Tz increases up to��1� 108 Pa. This phenomenon comes from high genera-tion of negative piezoelectric potential, by increase of thecompressive strain, which strongly repelled the free elec-trons from ZnO to outside at the interface. It can beconcluded from the simulation results that depletion widthof p–n junction is one of the key factor for high performancesemiconducting piezoelectric nanogenerator.

Conclusion

In summary, ZnO NWs-based NG performance has beenimproved four times, by using a p-type P3HT polymer as asurface passivation agent on the surface of as-grown ZnONWs. The high performance piezoelectric semiconductingp–n junction-based NG was achieved by controlling thedepletion width and energy band under mechanical strain,as well as native defects in the as-grown ZnO NWs. Theoutput voltage and output current density of pure ZnO NGwere observed at 22 mV and 200 nA/cm2, respectively,under a vertical compressive force of 0.8 kgf. By addingP3HT polymer, the output voltage and current increasedramatically up to a maximum value of 320 mV and950 nA/cm2, under application of the same vertical com-pressive strain. Using p-type polymer, the piezoelectricscreening effect caused by free electrons and surfacedefects in the ZnO NWs is significantly reduced, withoutany other treatment, such as thermal or plasma. Thevariation of piezoelectric electric potential with free carrierconcentration under different compressive strains for thecase of p-type P3HT-coated ZnO NW-based NG was carriedout using COMSOL multiphysics software simulations, anddiscussed in terms of the built p–n junction.

Acknowledgments

K.Y. Lee, J. Bae, and S.M. Kim contributed equally to thiswork. This work was financially supported by Basic ScienceResearch Program (2012R1A2A1A01002787, 2009-0083540)and Global Frontier Research Center for Advanced SoftElectronics (2013M3A6A5073177) through the NationalResearch Foundation (NRF) of Korea Grant funded by theMinistry of Science, ICT & Future Planning.

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Keun Young Lee a Ph.D. student under thesupervision of Prof. Sang-Woo Kim at Schoolof Advanced Materials Science and Engineer-ing, Sungkyunkwan University (SKKU). Hisresearch interests are fabrications and char-acterizations of piezoelectric and triboelectricnanagenerators for energy harvesting and theirapplications in self-powered devices.

Dr. Jihyun Bae received her Ph.D. degree inTextile Technology and Management fromNorth Carolina State University in 2007. Shehad involved the textile electronic platformproject at Samsung Advanced Institute ofTechnology (SAIT). Her research interestsinclude the textile/fiber electronics, flex-ible device, flat panel display technology,and optic simulation for display.

Dr. SeongMin Kim received his Ph.D. degreefrom University of Cambridge, UK in 2009.He has worked in Samsung Advanced Instituteof Technology (SAIT). His research interestsinclude multi-physics modeling/simulation par-ticularly for piezo-phototronic devices andtriboelectric nanogenerators.

Ju-Hyuck Lee is a Ph.D. student under thesupervision of Prof. Sang-Woo Kim at SKKUAdvanced Institute of Nano Technology(SAINT), Sungkyunkwan University (SKKU).His Ph.D. work is based on the fabricationsand characterizations of piezoelectric andpyroelectric nanagenerators for self-poweredbody implantable devices.

Gyu Cheol Yoon completed his Masterdegree under the supervision of Prof.Sang-Woo Kim from the School of AdvancedMaterials Science & Engineering at Sung-kyunkwan University (SKKU) in 2014. Hisresearch interests are fabrications and char-acterizations of hybrid energy harvestersuch as solar-piezoelectric nanogenerators.

Dr. Manoj Kumar Gupta received his Ph.D.degree from University of Delhi, India in2011 under the supervision of Prof. BinayKumar. Presently, he is working with Profes-sor Sang-Woo Kim as a postdoctoralresearcher at School of Advanced MaterialsScience and Engineering, SungkyunkwanUniversity (SKKU). His current researchareas are nano-materials synthesis and fab-rication of energy harvesting nanodevices

such as piezoelectric, pyroelectric, triboelectric and hybrid nano-generators.

Dr. Sungjin Kim received his Ph.D. degreefrom Seoul National University in 2010.From 2004, he has worked in SamsungElectronics as an advanced researcher. Hisresearch interests are computational andcomputing science for organic photo-elec-tronics, RF radio communications, scientificcomputing algorithms and tools. The con-vergence between chemistry theory andinformation technology for advanced mate-

rial simulations is the most recent interesting area.

Dr. Hyeok Kim studied Physics and Mathe-matics with minor from Korea AdvancedInstitute of Science and Technology (KAIST)then electrical engineering from SeoulNational University, where he obtainedB.S. and M.S., respectively. He performedthe research focused on organic electronicdevices, such as field-effect transistors anddiodes, during his Ph.D. from University ofParis 7 with Prof. Gilles Horowitz. After-

wards, he joined Samsung Advanced Institute of Technology (SAIT)and now works in Korea Institute of Science and Technology (KIST).His research interests include flexible optoelectronic devices andnanogenerators by using device modeling and simulation.

173Depletion width engineering via surface modification

Dr. Jongjin Park received his Ph.D. degreefrom Korea Advanced Institute of Scienceand Technology (KAIST) in 1999. From 2002to present, he has worked for SamsungAdvanced Institute of Technology (SAIT) asa master of materials and device. His cur-rent research program is wearable devicefor mobile healthcare using functionalnanofibers. He has published over 60 arti-cles and holds 260 patents.

Dr. Sang-Woo Kim is now Associate Profes-sor in School of Advanced Materials Science& Engineering and SKKU Advanced Instituteof Nanotechnology (SAINT), and SKKU YoungFellow at Sungkyunkwan University (SKKU).He received his Ph.D. from Kyoto Universityin Department of Electronic Science andEngineering in 2004. Prof. Kim pioneeredthe realization of large-scale transparentflexible piezoelectric nanogenerators for

harvesting mechanical energy in nature for self-powering of lowpower-consuming portable and body-implanted electronics. Hisrecent research interest is focused on piezoelectric/triboelectricnanogenerators, piezophototronics, and 2D layered materialsincluding graphene, h-BN, MoS2, etc.