Hexagonal boron nitride assisted growth of stoichiometric Al2O3 dielectric on graphene for triboelectric nanogenerators

Hexagonal boron nitride assisted growth of stoichiometric Al2O3 dielectric on graphene for triboelectric nanogenerators

Nano Energy (2015) 12, 556–566 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/nanoenergy RAPID COMMUNICATION ...

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Nano Energy (2015) 12, 556–566

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

RAPID COMMUNICATION

Hexagonal boron nitride assisted growth of stoichiometric Al2O3 dielectric on graphene for 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 a

SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), SKKU-Samsung Graphene Center, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea b School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea c NUSNNI-NanoCore, National University of Singapore (NUS), 117580, Singapore d Samsung 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 2015 Available online 24 January 2015

KEYWORDS

Abstract

Hexagonal boron nitride nanosheets; Graphene; Atomic layer deposition; Dielectric Al2O3; Triboelectric nanogenerator

Here we demonstrate the deposition of a high-k dielectric material on graphene using hexagonal boron nitride (h-BN) nanosheets as a buffer layer. The presence of an h-BN layer on 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 explanation 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 of Al2O3 for the fabrication of graphene-based devices. Finally, triboelectric nanogenerators (TNGs) based on both Al2O3/h-BN/graphene and Al2O3/graphene structures are demonstrated for 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

n Corresponding author at: SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), SKKUSamsung Graphene Center, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea. Tel.: +82 31 290 7352. E-mail address: [email protected] (S.-W. Kim).

http://dx.doi.org/10.1016/j.nanoen.2015.01.030 2211-2855/& 2015 Elsevier Ltd. All rights reserved.

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557 generation under a mechanical friction, while no significant electric power output from the Al2O3/graphene-based TNG is obtained, indicating high charge storage capacity of the dielectric Al2O3 layer on h-BN. & 2015 Elsevier Ltd. All rights reserved.

Introduction Graphene has been actively explored for future electronic applications owing to a unique combination of electrical, mechanical, and optical properties [1–4]. A seamless integration of graphene with a high-k dielectric material has been regarded as a very important issue for the realization of graphene-based electronic and energy devices. However, depositing a uniform film of high-k dielectric material on graphene without damaging the graphene is challenging. Ideally, a uniform film of a few nanometers thick high-k material with a stable stoichiometry and no pinholes is highly desirable for the realization of high performance graphenebased top-gated switching devices and graphene-based energy harvesting devices such as transparent flexible triboelectric nanogenerators (TNGs) [5–8]. However, the formation of a good-quality high-k dielectric layer on graphene is not straightforward due to the strikingly different chemical natures of these two materials [9]. Nevertheless, attempts have been made to integrate high-k dielectric materials on graphene using physical and chemical vapor deposition (CVD), as well as atomic layer deposition (ALD). All these methods, however, inevitably create defects in the carbon lattice monolayer, resulting in a poor and non-uniform graphenedielectric interface. Hence, nucleating and growing a uniform thin layer of high-k dielectrics on graphene has proven to be rather challenging. Although previous studies reported that a dielectric layer of reasonable quality can be grown using a modified ALD process, the surface functionalization of graphene should be introduced for the possible growth of oxide dielectrics on graphene [10–16]. However, these methods may change the unique properties of graphene or physically cause a great deal of damage to the graphene during the process. For instance, the functionalization process can potentially introduce undesired impurities or can result in the breaking of the chemical bonds in the graphene lattice, subsequently leading to significant degradation of carrier mobility [17]. Alternatively, the introduction of a polymer buffer layer prior to high-k deposition mitigates the potential damage to the graphene lattice; however, the unstable characteristics of the polymer is an issue in the durability of such top-gated graphene-based switching devices [18,19]. Furthermore, it cannot be easily applied for the integration of ultrathin high-k dielectrics with graphene on a large-scale. Herein, we report a simple approach for the formation of damage-free, high-quality large-scale uniform formation of Al2O3 with a balanced stoichiometry on a CVD-grown layered hexagonal boron nitride (h-BN)/pristine graphene. We believe h-BN plays a crucial role in this structure. Although h-BN is an insulating material, it cannot be utilized for an effective dielectric material due to its low dielectric constant [20]. Both simulations and experimental studies

are carried out to verify the suitability of h-BN for the growth of good quality high-k Al2O3. The present study was undertaken because h-BN is atomically flat and has almost the same hexagonal structure as graphene. Further, it possesses many attractive properties, such as high thermal stability, high mechanical strength, large thermal conductivity, a lattice constant similar to that of graphene, and is free of dangling bonds and charge traps with sp2 bonding configurations [21–26]. Thus, a good-quality oxide layer such as Al2O3 can be deposited on h-BN using an ALD process due to 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 passivation layer for reducing carrier scattering and minimizing the degradation of carrier mobility in top-gated graphene-based switching devices. More importantly, it is expected that this Al2O3/h-BN/ graphene multi-layered structure with a very thin total thickness can be utilized for high-performance transparent flexible TNGs due to the high charge storage capacity of Al2O3 compared to that of pristine graphene. However, limited studies have been carried out on graphene-based TNGs [5] with high transparency and flexibility until now. In this regard, we demonstrate TNGs based on both Al2O3/hBN/graphene and Al2O3/graphene structures to further confirm the importance of h-BN for synthesizing highquality Al2O3 on graphene.

Results and discussion To confirm the importance of h-BN as a buffer layer for facilitating the growth of a good-quality high-k material, a comparative study of the properties of both h-BN and graphene-supported Al2O3 film was carried out. Both h-BN and graphene nanosheets were grown using CVD with copper as a catalyst. The information relevant to the experimental conditions for the growth of h-BN nanosheets is provided as supplementary data (see Figure S1, Supporting Information). The CVD-grown BN nanostructures were characterized using optical microscopy and Raman spectroscopy (Witec, alpha-300M, 532 nm, Ar + ion laser). The optical image and Raman spectrum of the CVD-grown BN nanostructures are given in the Supporting Information (Figure S2a and b). The appearance of a peak at approximately 1366 cm 1 in the Raman spectrum indicates the B–N vibration mode (E2g), which is the signature of h-BN formation [29]. ALD was utilized for Al2O3 deposition, as it offers high film quality with the possibility of controlling the number of layers. Trimethylaluminum (TMA) and de-ionized water (H2O) in the gaseous state were employed as precursor materials for the growth of Al2O3, and the ALD process, leading to Al2O3 deposition, which is schematically shown in Figure S3.

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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.

Transmission electron microscopy (TEM) was performed to characterize the Al2O3/h-BN/SiO2 layered nanostructure. A high-resolution TEM micrograph of the Al2O3/h-BN/SiO2 (crosssectional view) is presented in Figure 1a, which clearly shows that the ALD-deposited Al2O3 layer possesses a uniform thickness with a flat and featureless top surface. The interface of the ALD-deposited Al2O3 and CVD-grown h-BN is very smooth and free from any irregularity. The thickness of the Al2O3 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 is quite non-uniform and contains many pinholes (Figure 1b). Thus, the experimental results show that the growth of goodquality high-k oxide materials is favorable when using h-BN in comparison to bare graphene. We next sought to unravel the possible mechanism underlying these results. To understand the phenomenon more clearly, simulation studies based on density functional theory were performed using the Vienna Ab initio simulation package for the interactions at the Al2O3/h-BN and Al2O3/graphene interfaces. The projector-augmented-wave pseudo-potentials and the planewave basis set with a kinetic cutoff energy of 400 eV were used.

For the exchange correlation energy, the generalized gradient approximation in the formulation of the Perdew, Burke and Ernzerh of function was used. All geometries were optimized until the forces acting on each atom converged to within 0.02 eV/Å and the spin polarized effect had been considered. Graphene and h-BN sheets with 4  4 supercells were considered. The neighboring sheets were separated in the direction perpendicular to the surface by a Monk-Host Pack mesh with a 9  9  1 k-point grid. Dipole corrections were included in our calculations to correct the dipole interaction between periodic images. To investigate the interactions of the Al–O layer on graphene and h-BN sheets, we first studied the adsorption properties of Al and O atoms on graphene and h-BN using a first-principles calculation. On the h-BN sheet, O atoms are mainly adsorbed at the bridge site with a chemisorption energy of 2.13 eV (Figure 2a–c), while Al atoms are physisorbed with a physisorption energy of approximately 0.1 eV. In this case, Al atoms are expected to be floating on the h-BN sheet. From these results, one can see that in the initial deposition process of the Al–O layer on h-BN sheets, a local Oadsorbed region or O-layer on the sheets can be generated

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Figure 2 (a)–(c) Ab initio simulation results of the Al2O3 deposition mechanism on h-BN showing the adsorption positions of the O atoms. (d)–(f) Ab initio simulation results of the Al2O3 deposition mechanism on graphene, indicating the adsorption positions of O 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 of pristine graphene and an ALD-deposited Al2O3 thin film on graphene, respectively.

first (Figure 1c–e). Then, the Al atoms can approach, and be adsorbed 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 properties of Al atoms on the O-adsorbed region on the h-BN sheets were investigated.

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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.

For the O-adsorbed structures on the sheets, one-, two-, and three-O atom-adsorbed structures are considered, as shown in Figure 2a–c. In the O-adsorbed structures, O atoms are favored at the second neighbor sites, which were reported in the previous study [30]. When Al atoms approach the O-adsorbed structures on the h-BN, the Al–O molecular structures on the O-adsorbed structure of the hBN sheet remain chemically bonded (Figure 1c–e), indicating the bonding stability between the Al–ON molecules and h-BN sheets. As a result, a very flat Al2O3 layer is formed, which can be observed in the TEM micrograph in Figure 1a. The formation of the chemical bond between the Al–O molecules and the h-BN sheet may be due to the similar ionic bonding properties of the Al +/O- bond and the B+ /N- bond [27,28]. On the graphene sheet, O atoms are mainly adsorbed at the bridge site between the C–C bonds with a chemisorption energy of 2.36 eV (Figure 2d–f). This is similar to the h-BN sheet, which is in agreement with the previous study [30]. However, Al atoms are weakly adsorbed on graphene with an adsorption energy of 0.68–0.73 eV. In particular, the adsorption energy of Al atoms on

graphene is similar, within 0.05 eV, regardless of the adsorption site. This result indicates that Al atoms can be mobile on graphene. In this case, the breaking of the chemical bond between O and C is caused by one O- and three O-adsorbed structures, resulting in inert Al–O and Al-O3 molecular structures over the graphene (Figure 1f–h). Importantly, the adsorption energy of Al–O is greater than that of C–O; thus, when Al atoms adsorb with O atoms, Al–O bonding occurs some distance from the graphene surface. In some cases, however, Al–O bonding remains on the graphene surface, and a bond with the graphene is maintained. For this reason, when an attempt is made to deposit an Al2O3 layer, AlO(OH) or Al(OH)x bonding occurs instead of Al–O bonding. This results in the formation of a nonflat Al2O3 layer on the graphene surface (Figure 1b). The main reason for the difficulty involved in depositing Al2O3 onto the graphene surface is that graphene has sp2 bonding with covalent bonding between C–C, which differs from h-BN. Figure 3a and b presents a non-contact AFM image of pristine h-BN after Al2O3 deposition. The ALD-deposited Al2O3 film on the h-BN was almost 10 nm thick and had a root mean square

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Figure 5 Raman spectra of bare graphene and ALD-deposited Al2O3 on graphene with h-BN (red) and without h-BN (blue). The inset depicts the defect peak position. (b) An enlarged view of the blue dotted rectangle is shown in Figure 5(a). (c) C 1s spectrum of pristine 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 is 0.67:0.13:0.20. (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) Output voltage as a result of triboelectrification between Al2O3 (on h-BN/graphene) and graphene. (b) Switching polarity testing result of the 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).

(RMS) surface roughness of 0.754 nm, while the RMS roughness of the bare h-BN was 0.527 nm. In contrast, a similar 10 nmthick ALD-deposited Al2O3 film on graphene had a rather poor film surface with a roughness of 2.569 nm, while the roughness of the pristine graphene was 0.680 nm (Figure 3c and d). The ALD-deposited Al2O3 film on graphene had many pinholes and lacked uniformity. Thus, the obtained results led to the conclusions that Al2O3 nucleation directly onto graphene is not suitable due to the absence of favorable surface sites [9] and that the surface morphology and the roughness of h-BN and graphene do not play significant roles. Rather, the chemical structure of the substrate materials is very important for the growth of good-quality Al2O3 film. To further clarify the reason why Al2O3 nucleation on hBN is preferable to nucleation on graphene, X-ray photoelectron spectroscopy (XPS) was performed to determine the effect of the surface chemical composition and bonding aspects 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 is assigned to the chemical binding state of oxidized Al [10,31]. This indicates that the Al–O bonding peak position was very close to that of the Al2O3 phase. The O 1s peaks at 530.6 eV originated from Al2O3. Figure 4c and d presents the XPS spectra of Al 2p and O 1s, respectively, which are recorded from the 10 nm-thick Al2O3 film on graphene. In this case, the Al 2p peak can be deconvoluted into two peaks. One of the prominent peaks at 74.4 eV corresponds to aluminum oxide hydroxide (AlOOH), and a rather weak

peak at 76.3 eV corresponds to Al2O3. Additionally, the O 1s is composed of three peaks. The first main peak, at 531.3 eV, corresponds to Al atoms in the O environment, indicating the formation of AlOOH. The second and third components, at 532.9 eV and 530.6 eV, correspond to H2O and Al2O3, respectively. This confirms that the chemical composition and bonding aspects do not favor the growth of Al2O3 on bare graphene. The effect of Al2O3 thickness on hBN was investigated by varying the number of ALD cycles such as 40 and 80 cycles. Even the thickness of the Al2O3 layer is thin and flat, and a non-pinhole layer is deposited on the h-BN surface (see Figure S6, Supporting Information). Also, the capacitance of the Al2O3/h-BN layer obtained after 120 cycles is 5.6 nF when the thickness is 10 nm and the dielectric constant is 9.46 as calculated from Figure S7. Another important aspect of the h-BN layer that should be emphasized is that it can work as a passivating layer for the graphene-based devices. In fact, graphene becomes damaged and oxidation may take place at the surface during the ALD process of Al2O3 deposition. The h-BN layer can be useful to prevent such effects and can play an important role in passivating graphene from Al2O3. To determine the passivating capability of the h-BN layer, Raman and XPS characterization were performed on the Al2O3/graphene and Al2O3/h-BN/ graphene layered structures. Figure 5a presents the Raman spectra of pristine monolayer graphene (black), Al2O3/h-BN/ graphene (red), and Al2O3/graphene (blue) over SiO2/Si. The analysis of the Raman data shows that the graphene monolayer remains intact in the case of Al2O3/h-BN/graphene, as evidenced by the ratio of the 2D/G peaks. The ratio of the D to G

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

peak shows no increase, indicating that the disorder in the graphene remains unaffected after the ALD process. Additionally, the positions of the 2D and G peaks are not shifted in these films. However, the Raman data in the case of Al2O3/ graphene clearly show an increase in the intensity of the defect band (D band), indicating the introduction of substantial defects in the graphene lattice (Figure 5b). This also means that the ALD deposition of Al2O3 damages the graphene, further reducing the number of favorable surface sites for Al2O3 nucleation directly onto graphene. Figure 5d and e presents the C 1s XPS spectra of Al2O3/hBN/graphene and Al2O3/graphene over SiO2/Si, respectively. The C 1s spectrum of the pristine graphene is shown in Figure 5c. The analysis of the XPS data confirmed the existence 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 is 0.65:0.12:0.23. Incidentally, few changes are observed in the ratio of C–C, C–O, and C˭O bonding, which is calculated to be 0.67:0.13:0.20 from the C 1s data of Al2O3/h-BN/graphene. This finding clearly shows that graphene is not damaged during the ALD deposition of Al2O3 because h-BN works as an effective shield. However, the ratio of C–C, C–O, and C˭O bonding is calculated as 0.57:0.25:0.18 in the case of Al2O3/ graphene. A substantial and observable change in the ratio of C–C and C–O bonding suggests that graphene is oxidized after the ALD deposition of Al2O3. From these results, it is thought that the h-BN layer is an effective passivating layer that protects graphene during the ALD process.

For further confirmation of the importance of h-BN for synthesizing high-quality Al2O3 on graphene, we fabricated TNGs based on both Al2O3/h-BN/graphene and Al2O3/graphene structures. Polyimide (PI) films were used as substrates to hold both the top and bottom layers as shown in Figure 6. The triboelectrification process takes place at the interface of graphene and Al2O3 under a vertical compression, resulting in the electric power generation. The output performance of the TNG based on the Al2O3/h-BN/graphene structure is presented in Figures 6 and S8. The output voltage and output current density of 1.2 V and 150 nA/cm2 are obtained, respectively, under a vertical compressive force of 1 kgf. Figure 6b shows the switching polarity testing results, confirming that the electric voltage output originated from the triboelectric power generation between Al2O3 and graphene. On the other hand, negligibly weak electric voltage output from the Al2O3/graphene-based TNG without h-BN was obtained as shown in Figure 6c and d. These results suggest that the high charge storage capacity of dielectric Al2O3 for effective triboelectrification is only available in the stoichiometric Al2O3 on h-BN/graphene rather than in the AlOOH on graphene. To investigate triboelectric behavior between top graphene and stoichiometric Al2O3, work function differences in between graphene and Al2O3 were studied by a Kelvin probe force microscopy (KPFM) technique (Park system, XE-100), as shown in Figure 7. The work functions of graphene and Al2O3 were determined to be 5.05 and 5.35 eV, respectively. This result can provide the power-generating principle of Al2O3/h-BN/

564 graphene-based TNG on the basis of triboelectrificationinduced electron transfer between two facing materials of graphene 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 is established between the two electrodes (Figure 8a). When a vertical compressive force is applied to the top surface of the device, the graphene and Al2O3 layer are rubbed together (Figure 8b). Triboelectric charges with opposite signs are generated because of electron injection in the graphene by induced thermal energy during the contact between the Al2O3 and graphene. Because of the differences in the work function of the Al2O3 and graphene, the electrons are injected from graphene (5.05 eV) to Al2O3 (5.35 eV) resulting in the generation of negative charges on the Al2O3 surface and positive charges on the graphene surface. As shown in Figure 8c, once the Al2O3 and graphene surfaces are separated from each other, to achieve equilibrium, electrons start to flow from the negative potential side (bottom graphene) to the positive potential side (top graphene), leading to an accumulation of electrostatically induced charges on the electrodes, resulting in a positive electrical signal. At equilibrium, no electrical signal is observed (Figure 8d). When an instantaneous vertical compression is applied to the TNG, the Al2O3 and graphene come into contact and short each other out. The dipole moment subsequently disappears or decreases in magnitude, and the electrostatic potential difference starts to diminish. Therefore, the reduced electric potential difference generates a flow of electrons from the top electrode side to the bottom electrode side that causes the accumulated charges to vanish, resulting in a negative electrical potential across the electrodes (Figure 8e).

Conclusions In conclusion, both the experimental and simulation results emphasize the importance of h-BN as a buffer layer in the fabrication of graphene-based electronic and energy devices for the deposition of high quality Al2O3 thin films. Owing to the existence of the partial ionic-covalent bonding between B and 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. In addition, the presence of an h-BN layer on top of the graphene surface effectively prevents possible oxidation during the ALD deposition of Al2O3. The different electrical power output results of TNGs arising from the triboelectrification between Al2O3 and graphene with/without h-BN undoubtedly confirm the importance of triboelectrification. Thus, these results suggest that the use of h-BN as a buffer layer for synthesizing high-k dielectric materials on graphene is very promising to improve the performance of graphenebased electronic and energy devices.

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

S.A. Han et al. Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT and Future Planning.

Appendix A.

Supporting information

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

References [1] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [2] D.B. Farmer, H.-Y. Chiu, Y.-M. Lin, K.A. Jenkins, F.N. Xia, P. Avouris, Nano Lett. 9 (2009) 4474–4478. [3] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81 (2009) 109–162. [4] P. Avouris, Nano Lett. 10 (2010) 4285–4294. [5] S. Kim, M.K. Gupta, K.Y. Lee, A. Sohn, T.Y. Kim, K.-S. Shin, D. Kim, S.K. Kim, K.H. Lee, H.-J. Shin, D.-W. Kim, S.-W. Kim, Adv. Mater. 26 (2014) 3918–3925. [6] F.-R. Fan, Z.-Q. Tian, Z.L. Wang, Nano Energy 1 (2012) 328–334. [7] D.Y. Kim, S. Lee, Z.-H. Lin, K.H. Choi, S.G. Doo, H. Chang, J.-Y. Leem, Z.L. Wang, S.-O. Kim, Nano Energy 9 (2014) 101–111. [8] B. Meng, W. Tang, X. Zhang, M. Han, W. Liu, H. Zhang, Nano Energy 2 (2013) 1101–1106. [9] X. Wang, S.M. Tabakman, H. Dai, J. Am. Chem. Soc. 130 (2008) 8152–8153. [10] B. Lee, S.-Y. Park, H.-C. Kim, K. Cho, E.M. Vogel, M.J. Kim, R.M. Wallace, J. Kim, Appl. Phys. Lett. 92 (2008) 203102. [11] B. Fallahazad, K. Lee, G. Lian, S. Kim, C.M. Corbet, D.A. Ferrer, L. Colombo, E. Tutuc, Appl. Phys. Lett. 100 (2012) 093112. [12] G. Lee, B. Lee, J. Kim, K. Cho, J. Phys. Chem. C 113 (2009) 14225–14229. [13] B.J. Kim, H. Jang, S.-K. Lee, B.H. Hong, J.-H. Ahn, J.H. Cho, Nano Lett. 10 (2010) 3464–3466. [14] B. Lee, G. Mordi, M.J. Kim, Y.J. Chabal, E.M. Vogel, R. M. Wallace, K.J. Cho, L. Colombo, J. Kim, Appl. Phys. Lett. 97 (2010) 043107. [15] V.K. Sangwan, D. Jariwala, S.A. Filippone, H.J. Karmel, J.E. Johns, J.M.P. Alaboson, T.J. Marks, L.J. Lauhon, M.C. Hersam, Nano Lett. 13 (2013) 1162–1167. [16] J.M.P. Alaboson, C.-H. Sham, S. Kewalramani, J.D. Emery, J.E. Johns, A. Deshpande, T. Chien, M.J. Bedzyk, J.W. Elam, M.J. Pellin, M.C. Hersam, Nano Lett. 13 (2013) 5763–5770. [17] Z.H. Ni, H.M. Wang, Y. Ma, J. Kasim, Y.H. Wu, Z.X. Shen, ACS Nano 2 (2008) 1033–1039. [18] I. Meric, C.R. Dean, A.F. Young, N. Baklitskaya, N.J. Tremblay, C. Nuckolls, P. Kim, K.L. Shepard, Nano Lett. 11 (2011) 1093–1097. [19] F. Xia, D.B. Farmer, Y.-M. Lin, P. Avouris, Nano Lett. 10 (2010) 715–718. [20] K.K. Kim, A. Hsu, X. Jia, S.M. Kim, Y. Shi, M. Dresselhaus, T. Palacios, J. Kong, ACS Nano 6 (2012) 8583–8590. [21] K. Watanabe, T. Taniguchi, H. Kanda, Nat. Mater. 3 (2004) 404–409. [22] K.H. Lee, H.-J. Shin, J. Lee, I.-Y. Lee, G.-H. Kim, J.-Y. Choi, S.-W. Kim, Nano Lett. 12 (2012) 714–718. [23] H.-J. Shin, B. Kumar, H.S. Kim, R. Bhatia, S.-H. Kim, I.-Y. Lee, H.S. Lee, G.-H. Kim, J.-B. Yoo, J.-M. Choi, S.-W. Kim, Angew. Chem. Int. Ed 53 (2014) 11493–11497. [24] J.-G. Kho, K.-T. Moon, J.-H. Kim, D.-P. Kim, J. Am. Ceram. Soc. 83 (2000) 2681–2683.

Triboelectric nanogenerators [25] A. Ramasubramaniam, D. Naveh, E. Towe, Nano Lett. 11 (2011) 1070–1075. [26] K. Watanabe, T. Taniguchi, T. Niiyama, K. Miya, M. Taniguchi, Nat. Photon. 3 (2009) 591–594. [27] H. Liu, K. Xu, X. Zhang, P.D. Ye, Appl. Phys. Lett. 15 (2012) 152115. [28] N. Ooi, A. Rairkar, L. Lindsley, J.B. Adams, J. Phys.: Condens. Matter 18 (2006) 97–115. [29] D.M. Hoffman, G.L. Doll, P.C. Eklund, Phys. Rev. B 30 (1984) 6051. [30] M. Topsakal, S. Ciraci, Phys. Rev. B 86 (2012) 205402. [31] S. Verdier, L.E. Quatani, R. Dedryvere, F. Bonhomme, P. Biensan, D. Gonbeau, J. Electrochem. Soc. 154 (2007) A1088–A1099. Sang A Han is pursuing her Ph.D. degree under the supervision of Prof. Sang-Woo Kim at SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU). Her research interests are the synthesis and characterizations of two-dimensional materials and their applications.

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

Tae-Ho Kim is a Ph.D. candidate under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science & Engineering. His research interests include synthesis of 2D material such as graphene and h-BN, and the fabrication and characterization of 2D based devices.

Wanchul Seung is a Ph.D. student under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU). His research interests are the fabrications and characterizations of piezoelectric and triboelectric nanogenerator energy harvesting and their applications in self-powered devices.

Seok Kyeong Lee received his Masters degree under the supervision of Prof. SangWoo Kim at SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU). He is currently with LG Chemicals, Co. LTD. His recent research topic is flexible and next generation batteries.

565 Sungho Choi is a Ph.D. candidate in Materials Science and Engineering from Sungkyunkwan University (SKKU) under the supervision of Prof. Hyoung Sub Kim. His current research interests are the characteristics analysis of atomic layer deposited high-k gate dielectrics on 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 researcher with Prof. Sang-Woo Kim at the School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), South Korea from 2011 to 2013. He is currently working at NUSNNI-NanoCore, NUS, Singapore. His current research areas are the fabrication of energy harvesting nanoelectronics devices such as solar cells, nanogenerators, hybrid devices, and 2D based devices. Dr. Ravi Bhatia is working as a Postdoctoral Fellow with Prof. Sang-Woo Kim at Sungkyunkwan University (SKKU), South Korea. He earned his doctorate degree from the Department of Physics, Indian Institute of Science (IISc) in 2012. During his Ph.D. tenure, he worked on the low temperature charge transport and magnetic properties of iron-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 & Staff Member at the Samsung Advanced Institute of Technology (SAIT) at Samsung Electronics, Co. LTD. She received her Ph.D. from Sungkyunkwan University (SKKU) in SKKU Advanced Institute of Nanotechnology (SAINT) in 2010. She joined SAIT in 2004. Her recent research interest is focused on the synthesis of two-dimensional nanomaterials such as graphene, hexagonal boron nitride, and transition metal dichalcogenide nanosheets and electronic devices.

Dr. Woo-Jin Lee is a Research Staff Member at the Samsung Advanced Institute of Technology (SAIT). He received his Ph.D. from Korea Advanced Institute of Science and Technology (KAIST) in Physics in 2009. After working as a postdoctoral researcher at the Korea Research Institute Standards Science (KRISS), he joined SAIT in 2011. His recent research focused on two-dimensional nanomaterials including graphene, hexagonal boron nitride nanosheets, and transition metal dichalcogenides (TMDC), etc.

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S.A. Han et al. Dr. SeongMin Kim received his Ph.D. degree from University of Cambridge, UK, in 2009. He is currently with the Samsung Advanced Institute of Technology. His research interests include multi-physics modeling/simulation, particularly for piezo-phototronic devices and triboelectric nanogenerators.

Prof. Hyoung Sub Kim received a Ph.D. degree in Materials Science and Engineering from Stanford University, Stanford, CA, in 2004. After receiving his Ph.D. degree, he worked as a Postdoctoral Fellow in Electrical Engineering at Stanford University. Currently, he is an Associate Professor of Advanced Materials Science and Engineering at Sungkyunkwan University, Suwon, Korea. His major research interests are atomiclayer-deposition of various high-k films on novel substrates and their device application. In addition, his recent interests include synthesis and characterization of twodimensional transition metal dichalcogenides for novel electronic device application. Dr. Jae-Yong Choi is the Vice President in Material Research Center of Samsung Advanced Institute of Technology (SAIT), Samsung Electronics. He received his Ph.D. from Korea Advanced Institute of Technology (KAIST) at the Department of Materials Science and Engineering in 1998. He joined Ames National Laboratory in the United States as a postdoctoral researcher in 1998 and started his work as a research scientist at SAIT in 1999. His current research interest is the design and growth of graphene and two-dimensional materials and their electronic, energy, and sensor applications.

Prof. Sang-Woo Kim is an Associate Professor in School of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). He received his Ph.D. from Kyoto University in Department of Electronic Science and Engineering in 2004. After working as a postdoctoral researcher at Kyoto University and University of Cambridge, he spent 4 years as an assistant professor at Kumoh National Institute of Technology. He joined the School of Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology (SAINT) at SKKU in 2009. His recent research interest is focused on piezoelectric/triboelectric nanogenerators, photovoltaics, and two-dimensional nanomaterials including graphene and hexagonal boron nitride nanosheets. Now he is an Associate Editor of Nano Energy and an Executive Board Member of Advanced Electronic Materials.