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RAPID COMMUNICATION
Piezoelectric nanogenerator with a
nanoforest structure
Myeong-Lok Seol,Ji-Min Choi,Jee-Yeon Kim,Jae-Hyuk Ahn,
Dong-Il Moon,Yang-Kyu Choi n
Department of Electrical Engineering,KAIST,291Daehak-ro,Yuseong-gu,Daejeon305-701,
Republic of Korea
Received30December2012;received in revised form11March2013;accepted23April2013
Available online7May2013
KEYWORDS
Nanogenerator;
Metal-assisted?che-
mical etching?(mac-
etch);
Energy harvesting;
Nanoforest;
Piezoelectric;
Barium titanate(BTO)
Abstract
Piezoelectric nanogenerators have been intensively developed in terms of their materials and
applications;however,only modest structural progress has been made due to limitations in the
growth mechanisms of nano-materials.In this work,a piezoelectric nanogenerator based on a
nanoforest structure is introduced.Through a metal-assisted chemical etching(mac-etch)
process,densely packed vertical nanowires and nanovoids are fabricated.The nanoforest
structure serves as a bottom electrode,which increases the interfacial area with a
subsequently deposited piezoelectric material,in this case barium titanate(BaTiO3).In the
proposed fabrication method,more various piezoelectric materials can be used for a piezo-
electric device compared to previously reported methods because the process relies not on the
growth mechanism but on the deposition technique.The proposed nanoforest structured
nanogenerator produces a 4.2times enhanced power density compared to the control
generator,which uses the same material but has a?at topology.The strong relationship
between the enhancement ratio and the nanoforest height is found.Both the experiment and
the simulation data present a consistent trend of a gradual increase with a short height and
saturation at a tall height.
&2013Elsevier Ltd.All rights reserved.
Introduction
Energy harvesting technology has been widely investigated
for future sustainable power sources.In particular,energy
harvesting using piezoelectricity attracts special interests
due to its abundant energy sources,scalability,and high
energy conversion ef?ciency.By virtue of the rapid devel-
opment of nanofabrication techniques,a piezoelectric
energy generator using a nanostructure,also known as a
nanogenerator,was introduced by Wang and Song[1].The
prototype nanogenerator utilized a vertically standing ZnO
nanowire array,while the tip of an atomic force microscope
(AFM)was used for the electrical contact and to apply
mechanical force.With further developments in recent 2211-2855/$-see front matter&2013Elsevier Ltd.All rights reserved.
https://www.doczj.com/doc/5e1085169.html,/10.1016/j.nanoen.2013.04.006
n Corresponding author.Tel.:+82423503477.
E-mail address:ykchoi@ee.kaist.ac.kr(Y.-K.Choi).
Nano Energy(2013)2,1142–1148
years,nanogenerators have been proven to be advanta-geous in terms of both performance and applications compared to conventional bulk-type generators.By employ-ing these nanostructures,the external applied stress was not only limited in the axial direction but was also activated along the radial direction,which results in a strain con?ne-ment effect[2,3].
The development of nanogenerators has been strongly related to material growth techniques.Vapor–solid–solid process,vapor–liquid–solid process,pulse laser deposition, and chemical approaches are usually used to fabricate the core nanostructure[4].Various materials such as ZnO,GaN, InN,and CdSe have been successfully grown and experi-mentally demonstrated as nanogenerators[5–11].Other applications beyond energy harvesting have also been intensively researched by means of hybrid integration with other components,such as chemical sensors,transistors,Li-ion batteries,and biosensors[12–15].Whereas notable advancements have been accomplished in terms of materi-als and applications,little progress has been made from a structural viewpoint.The vertically grown nanowire array, initiated from a prototype nanogenerator,remains the most widely used core nanostructure[1,5–12].An ostensible reason for using vertically standing nanostructures may be that they show very attractive performances metrics.How-ever,a more realistic reason is that the fabrication method of these structures is constrained by the bottom-up growth techniques.
As the nanogenerator evolves more aggressively,structural analysis increasingly becomes an essential issue.In2011,Cha et al.introduced a nanogenerator with a piezoelectric nanopore array structure that was created by means of a template-assisted fabrication process[16].An increased output piezopotential of approximately145%was demon-strated by conducting comprehensive experiments coupled with a simulation.Herein,we introduce a nanogenerator based on a nanoforest structure fabricated using metal-assisted chemical etching(mac-etch).Densely packed sub-10nm nanowires and nanovoids create a core nanostructure, which results in an extremely large interfacial area.After-wards,a piezoelectric material is deposited by physical vapor deposition(PVD)over the pre-existing nanoforest.Due to the nature of these deposition techniques,a greater variety of piezoelectric materials can be made available compared to the bottom-up growth process.It is noteworthy that the proposed nanogenerator was fabricated with neither photo-lithography nor plasma etching.Moreover,an extra step to remove the pre-existing template is unnecessary because the highly doped silicon nanoforest template itself works as a bottom electrode.
Experimental methods
Fabrication begins with a4in.p-type silicon wafer(Fig.1A). Heavily doped silicon was used because the silicon wafer will serve as the bottom electrode.The resistivity of the wafer is0.005ohm cm,which is low enough for charge conduction.A key process to make nanoforest morphology is the mac-etch process[17–19].To apply mac-etch to the silicon surface,Au of a thickness of6nm was deposited onto the silicon using a thermal evaporator.Such a thin Au layer cannot form a continuous?lm-like morphology.Instead,the layer forms an island-like morphology on the nanometer scale which is composed of naturally created Au clusters and voids among the clusters(Fig.1B).The island-like patterns of Au will act as a framework for the formation of the nanoforest.The morphology transition from a?lm-like pattern to the island-like pattern is governed by the thickness of the deposited Au.Hence,control of the Au ?lm thickness is crucial.The Au-coated silicon wafer was dipped into a wet etchant composed of HF,H2O2,and H2O (at a volume ratio of2:1:77).H2O2partially oxidized the silicon with the help of Au as a catalyst,and the
oxidized Figure1Schematics describing the fabrication procedures of the nanoforest generator.(A)Initial heavily doped Si substrate.
(B)Thin Au?lm of a thickness of6nm is deposited on the Si surface.The thin Au naturally forms an island-like morphology(C)Mac-etched silicon surface.The height of the nanoforest can be tuned by the mac-etch time.(D)BTO-deposited nanoforest surface with a thickness of100nm created by RF sputtering.(E)Device after the PMMA coating process.(F)Final nanoforest generator after the deposition of the top electrode,which is composed of Cr and Au with thicknesses of10nm and200nm,respectively.
1143 Piezoelectric nanogenerator with a nanoforest structure
part was etched away by HF.Because H2O2alone cannot oxidize silicon into silicon dioxide,only the Au-deposited part is etched away.Therefore,the voids among Au clusters were transformed into vertically standing nanowires,while the silicon under Au clusters was converted into voids. Consequently,they were patterned in the form of a nanoforest(Fig.1C).The sizes of the nanowires and nanovoids are in the sub-10nm range,which can scarcely be achieved using conventional photolithography due to the resolution limitation.
A BaTiO3(BTO)?lm with a thickness of100nm was then deposited onto the mac-etched silicon surface by RF sputtering(Fig.1D)[20].The source power was200W, the deposition pressure was3.0mTorr,and deposition time was30min.The mac-etched morphology acts as a template to form a BTO nanostructure.The BTO?lm covers the protruding nanoforest conformally.Control of the BTO?lm thickness is an important factor that governs the piezo-electricity in the proposed nanogenerator.When the thick-ness of the BTO?lm is too thin,the deposited BTO clusters cannot be linked to each other.Therefore,a continuous?lm cannot be created.Otherwise,when the thickness of the BTO is too thick,the density of voids inside the nanostruc-ture is severely decreased,which weakens the strain con?nement effect.After the deposition process,the BTO-deposited wafer was annealed at7001C for20min under O2 ambient.This annealing process converts the phase of the BTO from an amorphous to a poly-crystalline state,which is critical for enhancing the piezoelectricity.Detailed char-acterization to show the phase change of the BTO?lm with the aid of X-ray diffraction(XRD)and Raman spectroscopy is described in the supporting information.
Prior to the formation of the top electrode,a poly(methyl methacrylate)(PMMA)layer was coated onto the BTO-coated nanoforest surface(Fig.1E).It was deposited by spin-coating at4000rpm for45s and subsequently annealed at1701C for30min.This PMMA layer provides four distinct advantages.First,the layer serves as an insulating layer that prevents an undesirable piezoelectric screening effect caused by free electrons[21].Schottky barrier formation at the metal and dielectric interface is usually used for this purpose;however,the insertion of an insulating layer was found to be more straightforward and effective to block the ?ows of free electrons[7].Second,the layer serves as a protection layer that prevents electrical shorts.A small fraction of defects during the fabrication process can cause an electrical short between the top and bottom electrodes, which can critically degrade the output potential.Insertion of the PMMA layer can prevent such direct contact between electrodes.Third,the layer serves as a diffuser that omni-directionally dispenses any applied force.Owing to the vertically aligned nanostructures of the BTO over the nanoforest surface,the applied pressure is intensi?ed on only a speci?c protruding region.In this case,the insertion of a PMMA layer can redistribute the applied force axially and radially.Fourth,the layer provides planarization effect, which simpli?es subsequent processes.If a material for the top electrode is directly deposited onto the rough nanofor-est surface,it cannot be continuously connected.Moreover, electrical open can be happened in the worst case.
After the PMMA coating process,bi-metals for the top electrode were deposited using a thermal evaporator (Fig.1F).Cr of a thickness of10nm was initially deposited as an adhesion layer,and then Au of a thickness of200nm was deposited.Afterwards,the fabricated wafer was diced with a square shape and the top and bottom electrodes were connected with aluminum wire and silver paste.A poling process was subsequently conducted with an electric ?eld of100kV/cm for10h.
Results and discussions
Fig.2A–C show scanning electron microscope(SEM)images of the nanoforest structure,which is composed of vertically standing nanowires and nanovoids.The densely packed sub-10nm nanowires and nanovoids are inherited by the natu-rally formed island-like morphology of the thin Au?lm.Each shape and size of the nanowires appears random in detail; however,the overall density and height of the nanostruc-ture are uniform and can be accurately controlled.The density of the nanowires and nanovoids is determined by the initial thickness of the Au for the mac-etching process,and the height of the nanowires is determined by the mac-etch time.
The silicon nanoforest acts as a template for the subse-quently deposited BTO.After BTO deposition of100nm,the size of the nanovoids is reduced,whereas the density of the nanovoids is maintained(Fig.2D).When the thickness of the BTO layer is too thick,most of the nanovoids disappear, which reduces the strain con?nement effect.Otherwise,if the thickness is too thin,BTO clusters become disconnected from each other,causing the amount of piezopotential to be decreased.Through the optimization of the BTO?lm thick-ness,a clear strain con?nement effect can be achieved.The applied vertical pressures are redistributed not only along the axial direction(parallel with the height of the nanowire) but also along the radial direction(parallel with the diameter of the nanowire).
Fig.2E presents a cross-sectional SEM image of a working nanogenerator.The heavily doped silicon substrate and the silicon nanoforest structure are shown at the bottom.The sidewall and top of the nanoforest are completely covered by the BTO layer.This conformal step coverage was con?rmed by EDS image(Fig.2F).In the nanoforest region, both silicon and barium peaks are observed.This is evidence that the BTO?lm covers the sidewall of the nanowire.On the BTO-coated nanoforest,the PMMA layer and the bilayer of Cr and Au for the top electrode are placed.
Electrical measurement of the piezopotential created from the nanogenerator is conducted using specially designed stress applying equipment.A cylinder with a diameter of1.5cm repeatedly moves up and down to apply mechanical force onto the nanogenerator(Fig.3A).A control sample of a piezoelectric generator using?at BTO ?lm without a nanostructure was also prepared.The piezo-electricity was fairly compared under the same experimen-tal and measurement conditions.A process of applying downward force,holding it for1s,releasing it,and then holding it again for1s was iteratively carried out.Detailed experimental procedures are presented with a movie clip in the supporting information.Positive piezopotential was induced after the down-forcing process and negative
M.-L.Seol et al.
1144
piezopotential was in turn produced after the release of the force.
T o con ?rm the reliability of the piezopotential,a linear superposition test was performed (Fig.3B).The superposition test is a widely used experimental tool to determine whether the measured voltage comes from the piezopotential or from some other environmental effect [4,6].T wo nanogenerators producing peak piezopotential amounts of 0.17–0.20V are serially connected with the Al wire.The output peak piezo-potential of the serially connected nanogenerators is approximately 0.32–0.40V ,which is the doubled piezopoten-tial value arisen from the single nanogenerator .
Compared to the control device without the nanostructure,the nanoforest generator clearly produces higher output piezopotential.With further quantitative analyses,2.1times enhanced piezopotential was achieved by the nanoforest generator (Fig.3C).Similar to the output piezopotential,current density was also 2.0times enhanced (Fig.3D).There-fore,the nanoforest generator produces 4.2times enhanced output power density compared to the control generator .This increment ratio is high compared to the previously reported nanostructures such as a nanopore array [16].T o con ?rm the reproducibility and variability of the enhancement effect,total 14control devices and experimental devices are mea-sured (See supporting information).
In parallel,a simulation study of the performance of the nanogenerator was conducted using a commercialized simu-lator (COMSOL)[22].The material parameters of the electrodes,PMMA,and BTO are based on the initially de ?ned values.The structure is modeled as a BTO-coated nanorod array (Fig.4A).To simplify the simulation,a well-ordered nanorod was assumed.The radius of the nanorod and the rod-to-rod distance were set to 50nm and 200nm,respectively.The thickness of the BTO ?lm was set to 100nm and the step coverage for the sidewall was assumed to be 50%.The edge of each BTO-coated nanorod is connected to the edge of another;thus,the designed structure could be also considered as a nanovoid array.Although the designed structure differs from the fabricated structure in terms of its detailed morphology,the underlying concept of the structure is very similar .The piezopotential distribution is displayed as a color map after applying stress to the top electrode.Because the nanoforest template also works as the bottom electrode,all nanorods are electrically grounded as the reference voltage;hence,the relative piezopotential was extracted from the top of the BTO ?
lm.
Figure 2Microscope images during and after fabrication.(A)SEM image of the nanoforest silicon surface created with a mac-etching time of 2min with a bird's-eye view angle.(B)SEM image of a nanoforest silicon surface created with a mac-etching time of 8min.(C)SEM image of a nanoforest silicon surface from a top view angle.Au-deposited site before the mac-etch becomes nanovoids and the others uncovered by Au become vertical nanowires.(D)SEM image of a BTO-deposited nanoforest from a top view angle.The BTO thickness is 100nm.Voids still remain among the BTO clusters.(E)Cross-sectional SEM image of the ?nal nanoforest generator .(F)EDS mapping result of the BTO-coated nanoforest region.The red dots come from Ba and the green dots come from silicon.
1145
Piezoelectric nanogenerator with a nanoforest structure
Through a comparative study using simulation models and experimental measurements,the height dependency of the nanoforest was analyzed.After the peak piezopotential of the control device without the nanoforest was set to 1.0,the relative amounts of the peak potentials were plotted (Fig.4B)according to various heights.The normalized value of 1.0is corresponding to 53mV of real potential.Simulated potential value could not be exactly de ?ned because the potential is directly in ?uenced by material parameters and applied pressure,which are dif ?cult to be exactly de ?ned.Therefore,normalized potential is more appropriate for clearer comparison.Both the simulation and measurement data show an increasing trend from 0nm to 100nm,and both show a saturation trend beyond 100nm.The output piezopotential was 2.1times increased when 250nm of nanoforest height was used compared to the control device without the nanostructure.This increase and saturation trend can be explained with the amount of radial strain.When the nanorod height is too low,radial movement perpendicular to the stress direction is inactive;hence,the strain con ?nement effect tends to be negligible.As the height is increased,the radial movements are signi ?cantly increased.Therefore the strain con ?nement effect is enforced.
Exponential relationship provides the best ?tting with the both simulation and measurement data points.As the nanoforest height increases,peak piezopotential is expo-nentially increased.The ratio of increment gradually
decreases as the nanoforest height increases over 100nm.From the equation in the Fig.4B,k rad represents the sensitivity between the peak piezopotential and the nano-forest height.k rad with the simulation curve is higher than that of measurement curve,which implies that simulation overestimated the radial strain effect.The possible reason of the difference may come from the step coverage of the BTO deposition.In simulation,conformal step coverage of 50%was assumed;however ,actual step coverage is higher for the shorter nanoforest height due to the corner effect.When nanoforest height is shortened,the corner effect planarizes the surface and effect of the nanostructure is more decreased than the predicted value.Based on these results,it is inferred that a longer mac-etch time is preferred to maximize the piezopotential.However ,a very tall nanoforest can lead to severe variability and can degrade the endurance.Too high aspect ratio of the structure would cause instability for the external forces,so the crystallinity of BTO and silicon nanoforest can be damaged after the operations.Therefore,careful optimiza-tion is required on demand.
Conclusions
In summary,a nanogenerator with a nanoforest structure was proposed and its piezoelectric characteristics were analyzed.A silicon nanoforest was fabricated by the
mac-
Figure 3Experimental procedures and results.(A)Snapshots during the measurement.The circular cylinder repeatedly moves up and down to apply stress to the nanogenerator .The inset represents the output voltage after the down-forcing (left)and releasing (right)process.(B)Result of linear superposition test.Doubled output piezopotential is measured when two devices are serially connected.One division in y -axis represents 0.1V .(C)Measured voltage curve from the control nanogenerator with a ?at BTO ?lm (black line)and from the nanoforest nanogenerator with the nanostructured BTO ?lm (red line).(D)Measured current density curve from the control nanogenerator with a ?at BTO ?lm (black line)and from the nanoforest nanogenerator with the nanostructured BTO ?lm (red line).
M.-L.Seol et al.
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etch process and was used as the template for the nanos-tructure.The nanoforest generator had advantages in terms of both fabrication and performance.In terms of fabrica-tion,the proposed fabrication can accommodate a greater variety of piezoelectric materials because it is not con-strained by the bottom-up growth mechanism.From a performance perspective,densely packed nanovoids inside the BTO-coated nanoforest activated the strain con ?nement effect,which results in increased piezopotential.From the fabricated nanogenerator ,4.2times increased output power density was demonstrated compared to the bulk-type con-trol generator .The height effect of the nanoforest was comparatively analyzed using a simulator (COMSOL)and by means of experimental results.Both results showed an increasing trend with short nanowires and saturation with the tall ones.The analyses on the nanoforest generator proposed in this work will not only be directly adaptable for use in silicon chip-based energy harvesting applications but will also inspire the further structural evolution of a practical and ef ?cient piezoelectric nanogenerator.
Acknowledgments
This work was sponsored through grants from the National Research and Development Program under Grant 2012-
0001131for the development of biomedical function mon-itoring biosensors sponsored by the Korea Ministry of Education,Science and Technology (MEST).This work was also supported by the Center for Integrated Smart Sensors funded by the Ministry of Education,Science and Technology as Global Frontier Project (CISS-2012M3A6A6054187).
Appendix A.Supporting information
Supplementary data associated with this article can be found in the online version at https://www.doczj.com/doc/5e1085169.html,/10.1016/j.nanoen.2013.04.006.
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Figure 4Simulation analysis of the height dependency in the nanogenerator .(A)Simulation schematic describing the modeled structure and the output piezopotential.Red to blue represents positive to negative electric potential.(B)Simulation (red dots)and measurement (green dots)results of the relative piezo-potential intensity for various heights of the nanogenerator .Dotted curves are exponential trend lines of the data.
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Piezoelectric nanogenerator with a nanoforest structure
Myeong-Lok Seol received the B.S.and M.S.degrees from the Korea Advanced Insti-tute of Science and Technology (KAIST),Daejeon,Korea,in 2010and 2012,respec-tively,where he is currently working toward the Ph.D.degree.His current research interests include organic-silicon hybrid device and piezoelectric nanogenerator
.
Ji-Min Choi received the B.S.degree from the Korea Advanced Institute of Science and Technology (KAIST),Daejeon,Korea,in 2009,where he is currently working toward the Ph.D.degree.His research interests include ?exible electronics,nanofabrication technology ,and
bioMEMS.
Jee-Yeon Kim received the B.S.degree and M.S.degree from the KAIST ,Daejeon,Korea,in 2009and 2011,respectively.She is cur-rently working toward the Ph.D.degree in the Department of Electrical Engineering,KAIST .Her research interests include biosen-sors and nanowire
electronics.
Jae-Hyuk Ahn received the B.S.degree from the KAIST ,Daejeon,Korea,in 2007,where he is currently working toward the Ph.D.degree.His research interests include electrical biosensors and nanofabrication technology
.
Dong-Il Moon received the B.S.degree from the Kyungbook National University ,Daegu,Korea,and M.S.degree from the KAIST ,Daejeon,Korea in 2008and 2010,respec-tively.He is currently working toward the Ph.D.degree in the Department of Electrical Engineering,KAIST .His research interests include ?oating body cell memories ranging from device design to process development,simulation,and
characterization.
Yang-Kyu Choi received the B.S.and M.S.degrees from Seoul National University,Seoul,Korea,in 1989and 1991,respec-tively,and the Ph.D.degree from the Uni-versity of California,Berkeley ,in 2001.He is currently a Professor with the Department of Electrical Engineering,KAIST .He has authored or coauthored over 280papers and is a holder of 12U.S.patents and 99Korea patents.Dr .Choire-ceived the Sakrison Award for the best
dissertation from the Department of Electrical Engineering and Computer Sciences,University of California,in 2002.He was also the recipient of “The Scientist of the Month for July 2006”from the Ministry of Science and Technology in Korea.
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