Available online at www.sciencedirect.com

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Nano Energy (2015) 12, 556–566

http://dx.doi.org/12211-2855/& 2015 E

nCorresponding auSamsung Graphene C

E-mail address: k

RAPID COMMUNICATION

Hexagonal boron nitride assisted growth ofstoichiometric Al2O3 dielectric on graphenefor triboelectric nanogenerators

Sang A. Hana, Kang Hyuck Leeb, Tae-Ho Kimb, Wanchul Seungb,Seok Kyeong Leea, Sungho Choib, Brijesh Kumarc, Ravi Bhatiaa,Hyeon-Jin Shind, Woo-Jin Leed, SeongMin Kimd, Hyoung Sub Kimb,Jae-Yong Choid, Sang-Woo Kima,b,n

aSKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT),SKKU-Samsung Graphene Center, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of KoreabSchool of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU), Suwon 440-746,Republic of KoreacNUSNNI-NanoCore, National University of Singapore (NUS), 117580, SingaporedSamsung Advanced Institute of Technology (SAIT), Yongin 446-712, Republic of Korea

Received 14 October 2014; received in revised form 31 December 2014; accepted 16 January 2015Available online 24 January 2015

KEYWORDSHexagonal boronnitride nanosheets;Graphene;Atomic layer deposi-tion;Dielectric Al2O3;Triboelectricnanogenerator

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

thor at: SKKU Adventer, Sungkyunkimsw1@skku.edu

AbstractHere we demonstrate the deposition of a high-k dielectric material on graphene usinghexagonal boron nitride (h-BN) nanosheets as a buffer layer. The presence of an h-BN layeron top of the graphene facilitated the growth of high-quality Al2O3 by atomic layer deposition(ALD). Simulation results also support the experimental observations and provide an explana-tion for the suitability of h-BN as a buffer layer in terms of mixed ionic-covalent B–N bonding.Additionally, h-BN works as a protective shield to prevent graphene oxidation during ALD ofAl2O3 for the fabrication of graphene-based devices. Finally, triboelectric nanogenerators(TNGs) based on both Al2O3/h-BN/graphene and Al2O3/graphene structures are demonstratedfor further confirming the importance of h-BN for synthesizing high-quality Al2O3 on graphene.It was found that the Al2O3/h-BN/graphene-based TNG reveals meaningful electric power

015.01.030hts reserved.

anced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), SKKU-wan University (SKKU), Suwon 440-746, Republic of Korea. Tel.: +82 31 290 7352.(S.-W. Kim).

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generation under a mechanical friction, while no significant electric power output from theAl2O3/graphene-based TNG is obtained, indicating high charge storage capacity of the dielectricAl2O3 layer on h-BN.& 2015 Elsevier Ltd. All rights reserved.

Introduction

Graphene has been actively explored for future electronicapplications owing to a unique combination of electrical,mechanical, and optical properties [1–4]. A seamless integra-tion of graphene with a high-k dielectric material has beenregarded as a very important issue for the realization ofgraphene-based electronic and energy devices. However,depositing a uniform film of high-k dielectric material ongraphene without damaging the graphene is challenging.Ideally, a uniform film of a few nanometers thick high-kmaterial with a stable stoichiometry and no pinholes is highlydesirable for the realization of high performance graphene-based top-gated switching devices and graphene-basedenergy harvesting devices such as transparent flexible tribo-electric nanogenerators (TNGs) [5–8]. However, the formationof a good-quality high-k dielectric layer on graphene is notstraightforward due to the strikingly different chemicalnatures of these two materials [9]. Nevertheless, attemptshave been made to integrate high-k dielectric materials ongraphene using physical and chemical vapor deposition (CVD),as well as atomic layer deposition (ALD). All these methods,however, inevitably create defects in the carbon latticemonolayer, resulting in a poor and non-uniform graphene-dielectric interface. Hence, nucleating and growing a uniformthin layer of high-k dielectrics on graphene has proven to berather challenging.

Although previous studies reported that a dielectric layerof reasonable quality can be grown using a modified ALDprocess, the surface functionalization of graphene should beintroduced for the possible growth of oxide dielectrics ongraphene [10–16]. However, these methods may change theunique properties of graphene or physically cause a greatdeal of damage to the graphene during the process. Forinstance, the functionalization process can potentiallyintroduce undesired impurities or can result in the breakingof the chemical bonds in the graphene lattice, subsequentlyleading to significant degradation of carrier mobility [17].Alternatively, the introduction of a polymer buffer layerprior to high-k deposition mitigates the potential damage tothe graphene lattice; however, the unstable characteristicsof the polymer is an issue in the durability of such top-gatedgraphene-based switching devices [18,19]. Furthermore, itcannot be easily applied for the integration of ultrathinhigh-k dielectrics with graphene on a large-scale.

Herein, we report a simple approach for the formation ofdamage-free, high-quality large-scale uniform formation ofAl2O3 with a balanced stoichiometry on a CVD-grown layeredhexagonal boron nitride (h-BN)/pristine graphene. Webelieve h-BN plays a crucial role in this structure. Althoughh-BN is an insulating material, it cannot be utilized for aneffective dielectric material due to its low dielectricconstant [20]. Both simulations and experimental studies

are carried out to verify the suitability of h-BN for thegrowth of good quality high-k Al2O3. The present study wasundertaken because h-BN is atomically flat and has almostthe same hexagonal structure as graphene. Further, itpossesses many attractive properties, such as high thermalstability, high mechanical strength, large thermal conduc-tivity, a lattice constant similar to that of graphene, and isfree of dangling bonds and charge traps with sp2 bondingconfigurations [21–26]. Thus, a good-quality oxide layer suchas Al2O3 can be deposited on h-BN using an ALD process dueto its partial ionic-covalent B–N bonding characteristic,which differs from that of graphene [27,28]. Additionally,h-BN nanosheets can exploit the benefits of a passivationlayer for reducing carrier scattering and minimizing thedegradation of carrier mobility in top-gated graphene-basedswitching devices.

More importantly, it is expected that this Al2O3/h-BN/graphene multi-layered structure with a very thin totalthickness can be utilized for high-performance transparentflexible TNGs due to the high charge storage capacity ofAl2O3 compared to that of pristine graphene. However,limited studies have been carried out on graphene-basedTNGs [5] with high transparency and flexibility until now. Inthis regard, we demonstrate TNGs based on both Al2O3/h-BN/graphene and Al2O3/graphene structures to furtherconfirm the importance of h-BN for synthesizing high-quality Al2O3 on graphene.

Results and discussion

To confirm the importance of h-BN as a buffer layer forfacilitating the growth of a good-quality high-k material, acomparative study of the properties of both h-BN andgraphene-supported Al2O3 film was carried out. Both h-BNand graphene nanosheets were grown using CVD with copperas a catalyst. The information relevant to the experimentalconditions for the growth of h-BN nanosheets is provided assupplementary data (see Figure S1, Supporting Informa-tion). The CVD-grown BN nanostructures were characterizedusing optical microscopy and Raman spectroscopy (Witec,alpha-300M, 532 nm, Ar+ ion laser). The optical image andRaman spectrum of the CVD-grown BN nanostructures aregiven in the Supporting Information (Figure S2a and b). Theappearance of a peak at approximately 1366 cm�1 in theRaman spectrum indicates the B–N vibration mode (E2g),which is the signature of h-BN formation [29]. ALD wasutilized for Al2O3 deposition, as it offers high film qualitywith the possibility of controlling the number of layers.Trimethylaluminum (TMA) and de-ionized water (H2O) in thegaseous state were employed as precursor materials for thegrowth of Al2O3, and the ALD process, leading to Al2O3

deposition, which is schematically shown in Figure S3.

Figure 1 (a) TEM image (cross-sectional) of Al2O3 on h-BN/graphene/SiO2, revealing the flat and uniform surface of Al2O3 on h-BN.(b) TEM image (cross-sectional) of Al2O3 on graphene/SiO2, indicating the non-uniform surface and pinholes of Al2O3 on graphene.(c)–(e) Ab initio simulation results of the Al2O3 deposition mechanism on h-BN and (f)–(h) graphene, respectively.

S.A. Han et al.558

Transmission electron microscopy (TEM) was performed tocharacterize the Al2O3/h-BN/SiO2 layered nanostructure. Ahigh-resolution TEM micrograph of the Al2O3/h-BN/SiO2 (cross-sectional view) is presented in Figure 1a, which clearly showsthat the ALD-deposited Al2O3 layer possesses a uniformthickness with a flat and featureless top surface. The interfaceof the ALD-deposited Al2O3 and CVD-grown h-BN is verysmooth and free from any irregularity. The thickness of theAl2O3 layer is approximately 10 nm, and it has no pinholes.However, a high-resolution TEM micrograph of the Al2O3/graphene/SiO2 layer shows that the ALD-deposited Al2O3 isquite non-uniform and contains many pinholes (Figure 1b).Thus, the experimental results show that the growth of good-quality high-k oxide materials is favorable when using h-BN incomparison to bare graphene. We next sought to unravel thepossible mechanism underlying these results.

To understand the phenomenon more clearly, simulationstudies based on density functional theory were performedusing the Vienna Ab initio simulation package for the interac-tions at the Al2O3/h-BN and Al2O3/graphene interfaces. Theprojector-augmented-wave pseudo-potentials and the plane-wave basis set with a kinetic cutoff energy of 400 eV were used.

For the exchange correlation energy, the generalized gradientapproximation in the formulation of the Perdew, Burke andErnzerh of function was used. All geometries were optimizeduntil the forces acting on each atom converged to within0.02 eV/Å and the spin polarized effect had been considered.Graphene and h-BN sheets with 4� 4 supercells were consid-ered. The neighboring sheets were separated in the directionperpendicular to the surface by a Monk-Host Pack mesh with a9� 9� 1 k-point grid. Dipole corrections were included in ourcalculations to correct the dipole interaction between periodicimages.

To investigate the interactions of the Al–O layer ongraphene and h-BN sheets, we first studied the adsorptionproperties of Al and O atoms on graphene and h-BN using afirst-principles calculation. On the h-BN sheet, O atoms aremainly adsorbed at the bridge site with a chemisorptionenergy of 2.13 eV (Figure 2a–c), while Al atoms are physi-sorbed with a physisorption energy of approximately 0.1 eV. Inthis case, Al atoms are expected to be floating on the h-BNsheet. From these results, one can see that in the initialdeposition process of the Al–O layer on h-BN sheets, a local O-adsorbed region or O-layer on the sheets can be generated

Figure 2 (a)–(c) Ab initio simulation results of the Al2O3 deposition mechanism on h-BN showing the adsorption positions of the Oatoms. (d)–(f) Ab initio simulation results of the Al2O3 deposition mechanism on graphene, indicating the adsorption positions ofO atoms.

Figure 3 (a) and (b) AFM images of bare h-BN and an ALD-deposited Al2O3 thin film on h-BN, respectively. (c) and (d) AFM images ofpristine graphene and an ALD-deposited Al2O3 thin film on graphene, respectively.

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first (Figure 1c–e). Then, the Al atoms can approach, and beadsorbed on, the O-adsorbed region or O-layer. In this case,the adsorption properties of Al atoms on the O-adsorbed

region are important. Subsequently, the adsorption propertiesof Al atoms on the O-adsorbed region on the h-BN sheets wereinvestigated.

Figure 4 XPS spectra of Al2O3 on h-BN and graphene. (a) Al 2p and (b) O 1s spectra of Al2O3 (ALD-deposited) on an h-BN nanosheet.(c) Al 2p and (d) O 1s spectra of Al2O3 (ALD-deposited) on graphene.

S.A. Han et al.560

For the O-adsorbed structures on the sheets, one-, two-,and three-O atom-adsorbed structures are considered, asshown in Figure 2a–c. In the O-adsorbed structures, O atomsare favored at the second neighbor sites, which werereported in the previous study [30]. When Al atomsapproach the O-adsorbed structures on the h-BN, the Al–Omolecular structures on the O-adsorbed structure of the h-BN sheet remain chemically bonded (Figure 1c–e), indicatingthe bonding stability between the Al–ON molecules and h-BNsheets. As a result, a very flat Al2O3 layer is formed, whichcan be observed in the TEM micrograph in Figure 1a. Theformation of the chemical bond between the Al–O moleculesand the h-BN sheet may be due to the similar ionic bondingproperties of the Al+/O- bond and the B+/N- bond [27,28].

On the graphene sheet, O atoms are mainly adsorbed at thebridge site between the C–C bonds with a chemisorption energyof 2.36 eV (Figure 2d–f). This is similar to the h-BN sheet, whichis in agreement with the previous study [30]. However, Al atomsare weakly adsorbed on graphene with an adsorption energy of0.68–0.73 eV. In particular, the adsorption energy of Al atoms on

graphene is similar, within 0.05 eV, regardless of the adsorptionsite. This result indicates that Al atoms can be mobile ongraphene. In this case, the breaking of the chemical bondbetween O and C is caused by one O- and three O-adsorbedstructures, resulting in inert Al–O and Al-O3 molecular structuresover the graphene (Figure 1f–h). Importantly, the adsorptionenergy of Al–O is greater than that of C–O; thus, when Al atomsadsorb with O atoms, Al–O bonding occurs some distance fromthe graphene surface. In some cases, however, Al–O bondingremains on the graphene surface, and a bond with the grapheneis maintained. For this reason, when an attempt is made todeposit an Al2O3 layer, AlO(OH) or Al(OH)x bonding occursinstead of Al–O bonding. This results in the formation of a non-flat Al2O3 layer on the graphene surface (Figure 1b). The mainreason for the difficulty involved in depositing Al2O3 onto thegraphene surface is that graphene has sp2 bonding withcovalent bonding between C–C, which differs from h-BN.

Figure 3a and b presents a non-contact AFM image of pristineh-BN after Al2O3 deposition. The ALD-deposited Al2O3 film onthe h-BN was almost 10 nm thick and had a root mean square

Figure 5 Raman spectra of bare graphene and ALD-deposited Al2O3 on graphene with h-BN (red) and without h-BN (blue). The insetdepicts the defect peak position. (b) An enlarged view of the blue dotted rectangle is shown in Figure 5(a). (c) C 1s spectrum ofpristine graphene. Ratio of C–C:C–O:C˭O is 0.65:0.12:0.23. (d) C 1s spectrum of Al2O3 on graphene with h-BN. Ratio of C–C:C–O:C˭O is0.67:0.13:0.20. (e) C 1s spectrum of Al2O3 on graphene without h-BN. Ratio of C–C:C–O:C˭O is 0.57:0.25:0.18.

Figure 6 Electric power output performance of TNGs arising from triboelectrification between Al2O3 and graphene. (a) Outputvoltage as a result of triboelectrification between Al2O3 (on h-BN/graphene) and graphene. (b) Switching polarity testing result ofthe TNG of Figure (a). (c) Output voltage as a result of triboelectrification between AlOOH (on graphene) and graphene.(d) Switching polarity testing result of the TNG of Figure (c).

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Figure 7 Work function measurements of graphene and Al2O3 using KPFM. (a) Schematic diagram of the KPFM set up.(b) Experimentally measured work functions of graphene and Al2O3 in this work. Work function maps of graphene (c) and Al2O3 (d).

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(RMS) surface roughness of 0.754 nm, while the RMS roughnessof the bare h-BN was 0.527 nm. In contrast, a similar 10 nm-thick ALD-deposited Al2O3 film on graphene had a rather poorfilm surface with a roughness of 2.569 nm, while the roughnessof the pristine graphene was 0.680 nm (Figure 3c and d). TheALD-deposited Al2O3 film on graphene had many pinholes andlacked uniformity. Thus, the obtained results led to theconclusions that Al2O3 nucleation directly onto graphene isnot suitable due to the absence of favorable surface sites [9]and that the surface morphology and the roughness of h-BN andgraphene do not play significant roles. Rather, the chemicalstructure of the substrate materials is very important for thegrowth of good-quality Al2O3 film.

To further clarify the reason why Al2O3 nucleation on h-BN is preferable to nucleation on graphene, X-ray photo-electron spectroscopy (XPS) was performed to determinethe effect of the surface chemical composition and bondingaspects of Al2O3 nucleation on the h-BN and graphene.Figure 4a and b shows the XPS spectra of Al 2p and O 1s,respectively, which originated from the 10 nm-thick Al2O3

layer on h-BN. The symmetrical Al 2p peak at 74.1 eV isassigned to the chemical binding state of oxidized Al[10,31]. This indicates that the Al–O bonding peak positionwas very close to that of the Al2O3 phase. The O 1s peaks at530.6 eV originated from Al2O3. Figure 4c and d presents theXPS spectra of Al 2p and O 1s, respectively, which arerecorded from the 10 nm-thick Al2O3 film on graphene. Inthis case, the Al 2p peak can be deconvoluted into twopeaks. One of the prominent peaks at 74.4 eV correspondsto aluminum oxide hydroxide (AlOOH), and a rather weak

peak at 76.3 eV corresponds to Al2O3. Additionally, the O 1sis composed of three peaks. The first main peak, at531.3 eV, corresponds to Al atoms in the O environment,indicating the formation of AlOOH. The second and thirdcomponents, at 532.9 eV and 530.6 eV, correspond to H2Oand Al2O3, respectively. This confirms that the chemicalcomposition and bonding aspects do not favor the growth ofAl2O3 on bare graphene. The effect of Al2O3 thickness on h-BN was investigated by varying the number of ALD cyclessuch as 40 and 80 cycles. Even the thickness of the Al2O3

layer is thin and flat, and a non-pinhole layer is deposited onthe h-BN surface (see Figure S6, Supporting Information).Also, the capacitance of the Al2O3/h-BN layer obtained after120 cycles is �5.6 nF when the thickness is �10 nm and thedielectric constant is 9.46 as calculated from Figure S7.

Another important aspect of the h-BN layer that should beemphasized is that it can work as a passivating layer for thegraphene-based devices. In fact, graphene becomes damagedand oxidation may take place at the surface during the ALDprocess of Al2O3 deposition. The h-BN layer can be useful toprevent such effects and can play an important role inpassivating graphene from Al2O3. To determine the passivatingcapability of the h-BN layer, Raman and XPS characterizationwere performed on the Al2O3/graphene and Al2O3/h-BN/graphene layered structures. Figure 5a presents the Ramanspectra of pristine monolayer graphene (black), Al2O3/h-BN/graphene (red), and Al2O3/graphene (blue) over SiO2/Si. Theanalysis of the Raman data shows that the graphene monolayerremains intact in the case of Al2O3/h-BN/graphene, as evi-denced by the ratio of the 2D/G peaks. The ratio of the D to G

Figure 8 Power generation mechanism of the Al2O3/h-BN/graphene-based TNG. (a) Schematic diagram for the initial state of theTNG. The device is neutral when no force is applied. (b–e) Power-generating principle of the TNG in the presence/absence of avertical compressive force applied to the top surface of the device.

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peak shows no increase, indicating that the disorder in thegraphene remains unaffected after the ALD process. Addition-ally, the positions of the 2D and G peaks are not shifted inthese films. However, the Raman data in the case of Al2O3/graphene clearly show an increase in the intensity of thedefect band (D band), indicating the introduction of substantialdefects in the graphene lattice (Figure 5b). This also meansthat the ALD deposition of Al2O3 damages the graphene,further reducing the number of favorable surface sites forAl2O3 nucleation directly onto graphene.

Figure 5d and e presents the C 1s XPS spectra of Al2O3/h-BN/graphene and Al2O3/graphene over SiO2/Si, respectively.The C 1s spectrum of the pristine graphene is shown inFigure 5c. The analysis of the XPS data confirmed theexistence of C–C, C–O, and C˭O bonding in all of the samples.In the case of pristine graphene, the ratio of C–C:C–O:C˭O is0.65:0.12:0.23. Incidentally, few changes are observed in theratio of C–C, C–O, and C˭O bonding, which is calculated to be0.67:0.13:0.20 from the C 1s data of Al2O3/h-BN/graphene.This finding clearly shows that graphene is not damagedduring the ALD deposition of Al2O3 because h-BN works asan effective shield. However, the ratio of C–C, C–O, and C˭Obonding is calculated as 0.57:0.25:0.18 in the case of Al2O3/graphene. A substantial and observable change in the ratio ofC–C and C–O bonding suggests that graphene is oxidized afterthe ALD deposition of Al2O3. From these results, it is thoughtthat the h-BN layer is an effective passivating layer thatprotects graphene during the ALD process.

For further confirmation of the importance of h-BN forsynthesizing high-quality Al2O3 on graphene, we fabricatedTNGs based on both Al2O3/h-BN/graphene and Al2O3/graphenestructures. Polyimide (PI) films were used as substrates to holdboth the top and bottom layers as shown in Figure 6. Thetriboelectrification process takes place at the interface ofgraphene and Al2O3 under a vertical compression, resulting inthe electric power generation. The output performance of theTNG based on the Al2O3/h-BN/graphene structure is presentedin Figures 6 and S8. The output voltage and output currentdensity of 1.2 V and 150 nA/cm2 are obtained, respectively,under a vertical compressive force of 1 kgf. Figure 6b shows theswitching polarity testing results, confirming that the electricvoltage output originated from the triboelectric power genera-tion between Al2O3 and graphene. On the other hand, negligiblyweak electric voltage output from the Al2O3/graphene-basedTNG without h-BN was obtained as shown in Figure 6c and d.These results suggest that the high charge storage capacity ofdielectric Al2O3 for effective triboelectrification is only avail-able in the stoichiometric Al2O3 on h-BN/graphene rather thanin the AlOOH on graphene.

To investigate triboelectric behavior between top grapheneand stoichiometric Al2O3, work function differences in betweengraphene and Al2O3 were studied by a Kelvin probe forcemicroscopy (KPFM) technique (Park system, XE-100), as shownin Figure 7. The work functions of graphene and Al2O3 weredetermined to be 5.05 and 5.35 eV, respectively. This resultcan provide the power-generating principle of Al2O3/h-BN/

S.A. Han et al.564

graphene-based TNG on the basis of triboelectrification-induced electron transfer between two facing materials ofgraphene and Al2O3 with different work functions.

Initially the device is neutral in the absence of any pressure/force, and no charge, no electric potential difference isestablished between the two electrodes (Figure 8a). When avertical compressive force is applied to the top surface of thedevice, the graphene and Al2O3 layer are rubbed together(Figure 8b). Triboelectric charges with opposite signs aregenerated because of electron injection in the graphene byinduced thermal energy during the contact between the Al2O3

and graphene. Because of the differences in the work functionof the Al2O3 and graphene, the electrons are injected fromgraphene (5.05 eV) to Al2O3 (5.35 eV) resulting in the genera-tion of negative charges on the Al2O3 surface and positivecharges on the graphene surface. As shown in Figure 8c, oncethe Al2O3 and graphene surfaces are separated from eachother, to achieve equilibrium, electrons start to flow from thenegative potential side (bottom graphene) to the positivepotential side (top graphene), leading to an accumulation ofelectrostatically induced charges on the electrodes, resultingin a positive electrical signal. At equilibrium, no electricalsignal is observed (Figure 8d). When an instantaneous verticalcompression is applied to the TNG, the Al2O3 and graphenecome into contact and short each other out. The dipolemoment subsequently disappears or decreases in magnitude,and the electrostatic potential difference starts to diminish.Therefore, the reduced electric potential difference generatesa flow of electrons from the top electrode side to the bottomelectrode side that causes the accumulated charges to vanish,resulting in a negative electrical potential across the electro-des (Figure 8e).

Conclusions

In conclusion, both the experimental and simulation resultsemphasize the importance of h-BN as a buffer layer in thefabrication of graphene-based electronic and energy devicesfor the deposition of high quality Al2O3 thin films. Owing tothe existence of the partial ionic-covalent bonding between Band N, h-BN provides sites that are more favorable for Al2O3

nucleation, which results in the growth of high quality Al2O3

film with atomically flat surface and uniform thickness. Inaddition, the presence of an h-BN layer on top of thegraphene surface effectively prevents possible oxidationduring the ALD deposition of Al2O3. The different electricalpower output results of TNGs arising from the triboelectrifi-cation between Al2O3 and graphene with/without h-BNundoubtedly confirm the importance of triboelectrification.Thus, these results suggest that the use of h-BN as a bufferlayer for synthesizing high-k dielectric materials on grapheneis very promising to improve the performance of graphene-based electronic and energy devices.

Acknowledgments

This work was financially supported by Basic Science ResearchProgram (2012R1A2A1A01002787, 2009-0083540) and the Cen-ter for Advanced Soft-Electronics as Global Frontier Project(2013M3A6A5073177) through the National Research

Foundation of Korea (NRF) Grant funded by the Ministry ofScience, ICT and Future Planning.

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2015.01.030.

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Sang A Han is pursuing her Ph.D. degree underthe supervision of Prof. Sang-Woo Kim at SKKUAdvanced Institute of Nanotechnology (SAINT),Sungkyunkwan University (SKKU). Her researchinterests are the synthesis and characteriza-tions of two-dimensional materials and theirapplications.

Kang Hyuck Lee is pursuing his Ph.D. degreeunder the supervision of Prof. Sang-Woo Kim atthe School of Advanced Materials Science &Engineering, Sungkyunkwan University (SKKU).His research interests include synthesis of two-dimensional materials such as graphene, h-BN,and nanocrystalline graphene.

Tae-Ho Kim is a Ph.D. candidate under thesupervision of Prof. Sang-Woo Kim at Schoolof Advanced Materials Science & Engineer-ing. His research interests include synthesisof 2D material such as graphene and h-BN,and the fabrication and characterization of2D based devices.

Wanchul Seung is a Ph.D. student under thesupervision of Prof. Sang-Woo Kim at Schoolof Advanced Materials Science and Engi-neering, Sungkyunkwan University (SKKU).His research interests are the fabricationsand characterizations of piezoelectric andtriboelectric nanogenerator energy harvest-ing and their applications in self-powereddevices.

Seok Kyeong Lee received his Mastersdegree under the supervision of Prof. Sang-Woo Kim at SKKU Advanced Institute ofNanotechnology (SAINT), Sungkyunkwan Uni-versity (SKKU). He is currently with LGChemicals, Co. LTD. His recent research topicis flexible and next generation batteries.

Sungho Choi is a Ph.D. candidate in MaterialsScience and Engineering from SungkyunkwanUniversity (SKKU) under the supervision ofProf. Hyoung Sub Kim. His current researchinterests are the characteristics analysis ofatomic layer deposited high-k gate dielectricson III–V compound semiconductors.

Dr. Brijesh Kumar received his Ph.D.degree from Indian Institute of Technology,Delhi, in 2009 under the supervision of Prof.R.K. Soni. He was a postdoctoral researcherwith Prof. Sang-Woo Kim at the School ofAdvanced Materials Science and Engineer-ing, Sungkyunkwan University (SKKU), SouthKorea from 2011 to 2013. He is currentlyworking at NUSNNI-NanoCore, NUS, Singa-pore. His current research areas are the

fabrication of energy harvesting nanoelec-

tronics devices such as solar cells, nanogenerators, hybrid devices,and 2D based devices.

Dr. Ravi Bhatia is working as a PostdoctoralFellow with Prof. Sang-Woo Kim at Sung-kyunkwan University (SKKU), South Korea.He earned his doctorate degree from theDepartment of Physics, Indian Institute ofScience (IISc) in 2012. During his Ph.D.tenure, he worked on the low temperaturecharge transport and magnetic properties ofiron-filled multiwall carbon nanotube(MWCNT), and MWCNT based composite

systems. His current research interests are

focused on studying various aspects of two-dimensional materials.

Dr. Hyeon-Jin Shin is a Research & StaffMember at the Samsung Advanced Instituteof Technology (SAIT) at Samsung Electro-nics, Co. LTD. She received her Ph.D. fromSungkyunkwan University (SKKU) in SKKUAdvanced Institute of Nanotechnology(SAINT) in 2010. She joined SAIT in 2004.Her recent research interest is focused onthe synthesis of two-dimensional nanoma-terials such as graphene, hexagonal boron

nitride, and transition metal dichalcogenide

nanosheets and electronic devices.

Dr. Woo-Jin Lee is a Research Staff Memberat the Samsung Advanced Institute of Tech-nology (SAIT). He received his Ph.D. fromKorea Advanced Institute of Science andTechnology (KAIST) in Physics in 2009. Afterworking as a postdoctoral researcher at theKorea Research Institute Standards Science(KRISS), he joined SAIT in 2011. His recentresearch focused on two-dimensional nano-materials including graphene, hexagonal

boron nitride nanosheets, and transition

metal dichalcogenides (TMDC), etc.

S.A. Han et al.566

Dr. SeongMin Kim received his Ph.D. degreefrom University of Cambridge, UK, in 2009.He is currently with the Samsung AdvancedInstitute of Technology. His research inter-ests include multi-physics modeling/simula-tion, particularly for piezo-phototronicdevices and triboelectric nanogenerators.

Prof. Hyoung Sub Kim received a Ph.D.degree in Materials Science and Engineeringfrom Stanford University, Stanford, CA, in2004. After receiving his Ph.D. degree, heworked as a Postdoctoral Fellow in Electri-cal Engineering at Stanford University. Cur-rently, he is an Associate Professor ofAdvanced Materials Science and Engineeringat Sungkyunkwan University, Suwon, Korea.His major research interests are atomic-

layer-deposition of various high-k films on

novel substrates and their device application. In addition, hisrecent interests include synthesis and characterization of two-dimensional transition metal dichalcogenides for novel electronicdevice application.

Dr. Jae-Yong Choi is the Vice President inMaterial Research Center of SamsungAdvanced Institute of Technology (SAIT),Samsung Electronics. He received his Ph.D.from Korea Advanced Institute of Technol-ogy (KAIST) at the Department of MaterialsScience and Engineering in 1998. He joinedAmes National Laboratory in the UnitedStates as a postdoctoral researcher in 1998and started his work as a research scientist

at SAIT in 1999. His current research inter-

est is the design and growth of graphene and two-dimensionalmaterials and their electronic, energy, and sensor applications.

Prof. Sang-Woo Kim is an Associate Profes-sor in School of Advanced Materials Scienceand Engineering at Sungkyunkwan Univer-sity (SKKU). He received his Ph.D. fromKyoto University in Department of Electro-nic Science and Engineering in 2004. Afterworking as a postdoctoral researcher atKyoto University and University of Cam-bridge, he spent 4 years as an assistantprofessor at Kumoh National Institute of

Technology. He joined the School of

Advanced Materials Science and Engineering, SKKU AdvancedInstitute of Nanotechnology (SAINT) at SKKU in 2009. His recentresearch interest is focused on piezoelectric/triboelectric nano-generators, photovoltaics, and two-dimensional nanomaterialsincluding graphene and hexagonal boron nitride nanosheets. Nowhe is an Associate Editor of Nano Energy and an Executive BoardMember of Advanced Electronic Materials.