Nano Res. 2011, 4(5): 440–447

440

Selective Growth of ZnO Nanorods on SiO2/Si Substrates Using a Graphene Buffer Layer

Won Mook Choi1,§, Kyung-Sik Shin2,§, Hyo Sug Lee1, Dukhyun Choi3, Kihong Kim1, Hyeon-Jin Shin1, Seon-Mi Yoon1,

Jae-Young Choi1 ( ), and Sang-Woo Kim2 ( ) 1 Samsung Advanced Institute of Technology, Giheung, Yongin, Gyeonggi 446-712, Republic of Korea 2 School of Advanced Materials Science and Engineering, Sungkyunkwan (SKKU) Advanced Institute of Nanotechnology (SAINT), Center

for Human Interface Nanotechnology (HINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea 3 Department of Mechanical Engineering, Kyung Hee University, Yongin, Gyeonggi 446-701, Republic of Korea Received: 18 September 2010 / Revised: 14 December 2010 / Accepted: 27 December 2010 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT A promising strategy for the selective growth of ZnO nanorods on SiO2/Si substrates using a graphene buffer layer in a low temperature solution process is described. High densities of ZnO nanorods were grown over a large area and most ZnO nanorods were vertically well-aligned on graphene. Furthermore, selective growth of ZnO nanorods on graphene was realized by applying a simple mechanical treatment, since ZnO nanorods formed on graphene are mechanically stable on an atomic level. These results were confirmed by first principles calculations which showed that the ZnO–graphene binding has a low destabilization energy. In addition, it was found that ZnO nanorods grown on SiO2/Si with a graphene buffer layer have better optical properties than ZnO nanorods grown on bare SiO2/Si. The nanostructured ZnO–graphene materials have promising applications in future flexible electronic and optical devices. KEYWORDS Graphene, ZnO, nanorod, heterojunction, selective growth, solution

1. Introduction

Graphene, an atom-thick two-dimensional (2D) sheet of sp2-hybrized carbon, has attracted tremendous interests as a promising electronic material due to its unique electrical and mechanical properties that may enable a new generation of nanoelectronics and functional devices [1–4]. For these applications, there have recently been many attempts to control the electrical and interfacial properties of graphene such as functionalization and doping techniques. In addition,

semiconducting nanostructure/graphene junctions for three-dimensional (3D) nanoscale building blocks are very promising for future nanodevice applications based on the 3D integrated platform. However, there have been only a few reports of the synthesis and characterization of the hybrid structures of semicon- ducting one-dimensional (1D) and/or zero-dimensional (0D) nanomaterials with a 2D graphene layer [5–9]. In previous works, 1D and/or 0D nanomaterials were simply deposited onto graphene or were chemically linked to the defects of graphene [10, 11]. Meanwhile,

Nano Res. 2011, 4(5): 440–447 ISSN 1998-0124DOI 10.1007/s12274-011-0100-6 CN 11-5974/O4Research Article

§ These authors contributed equally to this work Address correspondence to Sangwoo Kim, kimsw1@skku.edu; Jaeyoung Choi, jaeyoung88.choi@samsung.com

Nano Res. 2011, 4(5): 440–447

441

1D semiconducting ZnO nanostructures have been intensively investigated for use in optoelectronic and photovoltaic device applications [12–14]. Thus, the 1D ZnO nanostructure/2D graphene heterojunction structure would be highly desirable for next-generation transparent, flexible electrical, and optical devices, including flexible photovoltaics, displays, and light emitters.

Recently, there have been reports of the growth of 1D ZnO nanorods and nanowires on graphene using high-temperature metal organic vapor phase epitaxy (MOVPE) and a photolithography-patterned ZnO seed layer based on a solution method, respectively [15, 16]. 1D ZnO nanostructure growth with the MOVPE process usually requires high growth temperatures over 400 °C, even if no catalyst is used, indicating difficulty in its use in fabricating flexible electronics based on plastic substrates. Although graphene with its extraordinary flexibility is expected to provide an excellent platform for emerging flexible devices, flexible substrates are still required for meaningful graphene- based flexible device applications. In this regard, an aqueous solution method can be regarded as a promising approach for growth of ZnO nanostructures on plastic substrates. Herein, we demonstrate the selective growth of ZnO nanorods on SiO2/Si substrates using a graphene buffer layer in a low temperature solution process without the need for a complicated patterning process. It is suggested that the selective growth of the ZnO nanorods using graphene is due to the strong ZnO–graphene interaction at the atomic/molecular level.

2. Experimental

2.1 Graphene preparation

Graphene was synthesized using thermal chemical vapor deposition (CVD) on a Ni-coated SiO2/Si substrate. A thin Ni layer 300 nm thick was first deposited onto a SiO2/Si substrate using an electron- beam evaporator. The Ni-coated substrate was placed in the thermal CVD chamber, the temperature was increased from room temperature to 900 °C, and the reduction process of the substrate was carried out in a hydrogen gas flow of 100 sccm for 10 min at 900 °C.

The gas mixing ratio of the CH4/H2 was optimized (10/100 sccm), and the growth time of the graphene was 10 min. After the growth was completed, the chamber was cooled to room temperature. The as-grown grap- hene sheet was separated from the underlying Ni layer using a Ni etchant (Transene, Type Ⅰ). The separated floating graphene sheet was transferred to SiO2/Si substrates using the fishing method. Wrinkles of grap- hene were formed during the high temperature CVD- growth process due to differences between the thermal expansion coefficients of the Si substrate, Ni catalyst, and graphene and during the transfer process in which the graphene floating on the water surface is transferred and dried on the substrate (as shown in Fig. 1).

To fabricate the patterned array of the graphene, the transferred graphene on the SiO2/Si substrate was first patterned with a photoresist (MicroChem, AZ1512) and then reactive ion etching (Oxford 100) was per- formed using O2 gas (100 W, 50 mTorr, and O2 50 sccm). The photoresist on the graphene patterns was then removed using photoresist stripper (MicroChem, AZ400T).

Figure 1 (a) FE-SEM and (b) AFM images of graphene transferred onto SiO2/Si substrates, respectively

Nano Res. 2011, 4(5): 440–447

442

2.2 ZnO nanorod growth

The prepared graphene samples were used as the platform for the growth of ZnO nanorods which were synthesized using the aqueous solution method. For seed-supported growth of ZnO nanorods, nanocrystal ZnO seed particles were prepared in a solution of 0.005 mol/L zinc acetate [Zn(CH3CO2)2] dissolved in ethanol at 90 °C for 15 min. The graphene-transferred SiO2/Si substrates were cleaned ultrasonically in acetone and de-ionized (DI) water for 5 min. The seed solution was then spin-coated onto graphene/SiO2/Si substrates at 2000 r/min for 1 min. Subsequently, the seed-coated substrates were thermally annealed at 300 °C for 5 min to remove the residual solvent. The procedures of spin coating and thermal annealing were repeated twice for ZnO nanorod growth with seed support. For the main growth, the graphene/SiO2/Si substrates with/without ZnO seeds were immersed in the same main growth solution consisting of 0.05 mol/L zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O), 0.025 mol/L hexamethylenetetramine (C6H12N4), and DI water. The main growth of ZnO nanorods was carried out at 90 °C for 90 min.

2.3 Computation methods

The first principles calculations based on density functional theory were performed using the Vienna ab initio simulation package (VASP) code for the structure optimizations. The graphene was composed of 24 C-atoms, and the ZnO surface slab had 36 Zn-atoms and 36 O-atoms (4 layers) and a wurtzite crystal structure. A quartz structure was used for the model of the SiO2 surface. The SiO2 slab had 24 Si-atoms and 48 O-atoms terminated by hydrogen atoms. In the case of heterojunction calculations, we added 15 Å of vacuum space, and the cell shape was changed during the structure optimizations. The binding energy was calculated as the total energy difference per unit area between the heterogeneous structure and the separated slab energies. The plane-wave cutoff energy was chosen to be 400 eV, and the k-points were sampled on 2 × 2 × 1 grids. The exchange-correlation interactions between electrons were described with the local density approximation (LDA), and ultra-soft pseudopotentials [17] were used for the description of the ion–electron

interactions. We used the conjugate gradient method for the geometric optimizations, and the optimization procedure was truncated when the residual forces for the relaxed atoms were less than 0.03 eV/Å.

2.4 Characterization

The samples were characterized by micro-Raman spectroscopy (Renishaw, RM1000-Invia) using 514 nm excitation and an optical microscope (Nikon, Elipse C200). Field-emission scanning electron microscopy (FE-SEM, Hitachi S-4700), atomic force microscopy (AFM, Veeco Dimension V), and transmission electron microscopy (TEM, FEI Tecnai G2) were also used to characterize the samples. Synchrotron X-ray diffraction (XRD) experiments were performed at the 3C2 beam line of Pohang Accelerator Laboratory (PAL). The wavelength of the synchrotron beam was 1.54 Å.

3. Results and discussion

The experimental procedures are illustrated in Fig. 2(a). The graphene used here was synthesized via CVD using methane as the carbon source and Ni film as the catalyst on a SiO2/Si substrate as described in our previous work [3, 4]. The Ni film was etched away using a Ni etchant, and then the graphene was transferred onto the substrate, and annealed at 250 °C for 10 min to give a strong adhesion. Using the transferred graphene, ZnO nanorods were synthesized using the aqueous solution method. The graphene sample was coated with a ZnO seed precursor and immersed into the growth reaction solution. The main growth of ZnO nanorods was carried out at 90 °C for 90 min. After the growth of the ZnO nanorods, the sample was treated briefly by ultrasonication in order to remove the unwanted bulk ZnO crystals which had formed in the growth solution. Full experimental details of the growth of the ZnO nanorods growth using the aqueous solution process are reported elsewhere [18, 20].

The Raman spectra of the prepared graphene and its optical microscope image are shown in Fig. 2(b). Due to the modulation of reflection, the thickness of graphene on the SiO2/Si substrate can be easily determined [21, 22]. In the Raman spectra of the thin

Nano Res. 2011, 4(5): 440–447

443

graphene layer (black line), the distinct G- and 2D- peaks are observed around 1600 cm–1 and 2700 cm–1, respectively with large peak intensity ratio of 2D to G(I2D/IG), which indicates the presence of monolayers or bilayers. Also, the thick graphene layer (red line) shows a small I2D/IG ratio and a broad 2D peak, characteristic of over 10 graphene monolayers. The average number of graphene layers used here was 6–7. Moreover, the negligible D-peak indicates that the graphene used here was of high quality without critical defects. An FE-SEM image of ZnO nanorods grown on graphene is shown in Fig. 2(c). The dense ZnO nanorods grew uniformly over large area on the graphene substrate, with the diameter of the nanorods being in the range 60–100 nm. Most of the ZnO nanorods were grown vertically on graphene (inset of Fig. 2(c)), while some of the ZnO nanorods were

grown at a tilted angle, which are likely due to the wrinkled structure of the graphene which is formed during the CVD-growth and transfer step or nonuni- form distribution of ZnO seeds [23, 24]. Although graphene has a highly hydrophobic surface leading to difficulty in the growth of other functional materials on it [25], the ZnO nanorods have been successfully grown on graphene via the aqueous solution process even at low temperature of 90 °C.

Figure 3 shows cross-sectional TEM images of the ZnO nanorods grown on graphene. The ZnO nanorods grown on the graphene layer are single-crystalline, as shown in the high-resolution (HR) TEM images and the fast Fourier transformation (FFT) patterns (Figs. 3(b)–3(e)). All of the spots in the FFT patterns taken from each graphene and ZnO nanorods are well aligned as 2D arrays. The pattern recorded from

Figure 2 (a) Schematic illustration of ZnO nanorod growth on graphene. (b) Raman spectra of graphene and optical microscope image ofgraphene transferred onto the SiO2/Si substrate. (c) FE-SEM image of ZnO nanorods grown on graphene via the aqueous solution process

Nano Res. 2011, 4(5): 440–447

444

Figure 3 (a) Cross-sectional bright-field TEM images of ZnO nanorods grown on graphene. (b) HR-TEM image of a single ZnO nanorod (area of the upper circle in image (a)). (c) Cross-sectional HR-TEM image of the ZnO nanorod-graphene interface (area of the lower circle in image (a)). The areas marked “P1” and “P2” indicate ZnO and graphene layers, respectively. (d) and (e) show fast Fourier transform (FFT) patterns of the regions “P1” and the “P2”, respectively

the ZnO nanorod was along the [1010] zone axis. The spacing between adjacent lattice planes in the ZnO was 0.26 nm, indicating the preferential growth of ZnO nanorods along the [0001] c-axis orientation. The crystalline lattice of the graphene was clearly visible by HR-TEM although the graphene layers were slightly damaged during the TEM sample preparation. The lattice spacing of 0.34 nm in the graphene is in good agreement with the interplanar distance of (0002) graphite.

For the crystallographic study of the ZnO–graphene heterojunction structures, synchrotron XRD measure- ments were carried out (Fig. 4). A strong diffraction peak at 2θ = 34.42° due to the ZnO (0002) plane is observed from which a lattice constant of 0.52 nm is estimated, and the clear graphene (0002) peak is also observed at 2θ = 26.42°. Very weak diffraction peaks corresponding to the ZnO (1010) and (1011) planes are also observed, which arise from ZnO nanorods tilted due to the wrinkled surface and crystalline defects of graphene or nonuniform distribution of ZnO seeds, consistent with the above FE-SEM and TEM results.

Furthermore, we performed the ZnO nanorod growth on a graphene pattern array. After the graphene was transferred onto the SiO2/Si substrate, followed the patterning process was carried out via photo- lithography followed by oxygen reactive ion etching. Thus, the dot-patterns of graphene were prepared

Figure 4 Synchrotron XRD results of ZnO nanorods grown on graphene

on the SiO2/Si substrate, and the blue color of the graphene patterns can be easily identified on the purple background of the SiO2 substrate as shown in Fig. 5(a), due to the reflective index difference between the two materials. After the growth of the ZnO nanorods with the solution process (Fig. 6(a)), the ZnO nanorods were grown densely on the graphene patterns, while they were grown less densely on the exposed SiO2 substrate. The sample was treated by ultrasonication for 1 min. It was observed (Fig. 6(b)) that a large number of the ZnO nanorods grown on the exposed SiO2 were detached from the substrate, while most of the ZnO nanorods on the graphene patterns remained adhered, confirming the mechanical stability of the adhesion between graphene and ZnO nanorods. The sample was further treated by ultra- sonication for 1 h sample for 1 h and the morpholo- gical changes investigated. The FE-SEM image of the sample after 1 h ultrasonication (Fig. 5(b)) reveals that

Figure 5 (a) Optical microscope image showing the periodic arrays of patterned graphene on the SiO2/Si substrate. (b) FE-SEM image of ZnO nanorods selectively grown mask-free on the graphene pattern arrays

Nano Res. 2011, 4(5): 440–447

445

Figure 6 FE-SEM images of the ZnO nanorods grown on the graphene patterns. (a) As-grown sample and (b) after ultrasonication treatment (1 min) of the as-grown sample

the ZnO nanorods remained on the graphene patterns without any significant density decrease, while almost no ZnO nanorods were left on SiO2. These results indicate that ZnO nanorods on graphene are mech- anically stable and are formed with a strong interaction at the atomic/molecular level.

We performed first principles calculations based on density functional theory using the VASP [26–28] code for structure optimization. Figure 7 shows the first principles calculation results of the interface between the graphene (0002) surface and the ZnO (0001) surface. For the ZnO–graphene heterojunction, the diagonal carbon (C)-atoms of graphene bind only with zinc (Zn)-atoms as shown in Fig. 7(a), and the C-atoms of graphene directly bind to the Zn-atoms of ZnO in the ZnO–graphene interface. For the ZnO–SiO2 junction, we modeled the quartz structure as the SiO2 structure, with the surface of the SiO2 terminated by oxygen (O)-atoms, placed in up and down sites. Only O-atoms in up sites bind with two Zn-atoms (bridge site), or

Figure 7 First principles calculation image of the atomic match between (a) ZnO (0001) and graphene (0002), and (b) ZnO and SiO2

the Zn-atoms bind with two O-atoms. However, the O-atoms of down sites cannot bind with Zn atoms and instead may create defect sites in the interface between the SiO2 and ZnO. In addition, the lattice mismatch between ZnO and SiO2 is larger than that between ZnO and graphene, and the SiO2 used here is amorphous having many defect sites. Therefore, these defect sites and the large lattice mismatch destabilize the energy of the ZnO–SiO2 interface.

The calculated results also show that the desta- bilization energy of the ZnO–graphene binding is 4.55 eV/nm2, while that for ZnO–SiO2 is 5.35 eV/nm2, indicating that ZnO has a more stable bond with graphene than it does with amorphous SiO2. These first principle calculation results explain why ZnO nanorods on SiO2 are easily removed by ultrasonication, while they formed a strong and stable bond and adhere on graphene even after 1 h of ultrasonication (as shown in Fig. 5(b)). The approach proposed herein provides a new way to fabricate the patterned arrays of ZnO–graphene 3D nanoscale building blocks using a lithography-free simple mechanical treatment due to the strong ZnO–graphene interaction, which is critical for device applications.

Figure 8 shows low temperature photoluminescence (PL) results obtained at 13 K using a He-Cd 325 nm laser line. As shown in the PL spectra, neutral donor- bound exciton (DoX) emission at 3.358 eV from the ZnO nanorods grown on the graphene/SiO2/Si substrate is sharper than that at 3.356 eV from the SiO2/Si substrate. This indicates that the ZnO nanorods on

Figure 8 PL spectra obtained from ZnO nanorods grown on graphene/SiO2/Si and ZnO nanorods grown on bare SiO2/Si at 13 K

Nano Res. 2011, 4(5): 440–447

446

the graphene/SiO2/Si have better crystal properties than the ZnO nanorods on the bare SiO2/Si.

In addition, the intensity ratios of DoX emission and deep level emission (R = IDX/IDEEP) are about 240 and 110 for that the ZnO nanorods on the graphene/SiO2/Si and the ZnO nanorods on the bare SiO2/Si, respectively (although the IDEEP is negligible compared with that of IDX) [29]. In this work, the PL came from the ZnO nanorods on both graphene and bare SiO2 formed under the same growth conditions. Thus, it can be concluded that the reason for the lower deep level emission from ZnO nanorods on graphene than that from those on bare SiO2 is the difference in crystallinity. Considering the first principle calculation results shown in the Fig. 7, it can be expected that fewer defect sites are created at the interface between the graphene and ZnO as compared to the interface between SiO2 and ZnO. From the PL results obtained in this work, it can be concluded that graphene is a promising buffer material for ZnO nanostructures with good optical properties.

3. Conclusions

We have demonstrated the selective growth of ZnO nanorods on SiO2/Si substrates using a graphene buffer layer taking advantage of the strong ZnO–graphene interaction confirmed by the first principles calculation study. The ZnO nanorods showed strong binding with the graphene, which enables the fabrication of patterned arrays of ZnO nanorod–graphene 3D nanoscale heterostructures without using lithographic techniques. These ZnO–graphene 3D heterojunctions are strongly desirable for next-generation flexible electrical and optical device applications including flexible photovoltaics, displays, and light emitters based on the 3D integrated platform.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Nos. 2010-0015035 and 2009-0077682) and also by the New and Renewable Energy of the Korea Institute of Energy Technology

Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 2009T100100614).

References

[1] Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666–669.

[2] Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based composite materials. Nature 2006, 442, 282–286.

[3] Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009, 457, 706–710.

[4] Reina, A.; Thiele, S.; Jia, X.; Bhaviripudi, S.; Dresselhaus, M. S.; Schaefer, J. A.; Kong, J. Growth of large-area single- and bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces. Nano Res. 2009, 2, 509–516.

[5] Xu, Y.; Liu, Z.; Zhang, X.; Wang, Y.; Tian, J.; Huang, Y.; Ma, Y.; Zhang, X.; Chen, Y. A graphene hybrid material covalently functionalized with porphyrin: Synthesis and optical limiting property. Adv. Mater. 2009, 21, 1275–1279.

[6] Kamat, P. V. Graphene-based nanoarchitectures. Anchoring semiconductor and metal nanoparticles on a two-dimensional carbon support. J. Phys. Chem. Lett. 2009, 1, 520–527.

[7] Lightcap, I. V.; Kosel, T. H.; Kamat, P. V. Anchoring semiconductor and metal nanoparticles on a two-dimensional catalyst Mat. storing and shuttling electrons with reduced graphene oxide. Nano Lett. 2010, 10, 577–583.

[8] Wang, X.; Tabakman, S. M.; Dai, H. Atomic layer deposition of metal oxides on pristine and functionalized graphene. J. Am. Chem. Soc. 2008, 130, 8152–8153.

[9] Kim, Y. T.; Han, J. H.; Hong, B. H.; Kwon, Y. U. Electrochemical deposition of CdSe quantum dot arrays on large-scale graphene electrodes using mesoporous silica thin film templates. Adv. Mater. 2010, 22, 515–518.

[10] Lee, B.; Park, S. Y.; Kim, H. C.; Cho, H.; Vogel, E. M.; Kim, M. J.; Wallace, R. M.; Kim, J. Conformal Al2O3 dielectric layer deposited by atomic layer deposition for graphene-based nanoelectronics. Appl. Phys. Lett. 2008, 92, 203102.

[11] Tong, L. M.; Li, Z. P.; Zhu, T.; Xu, H. X.; Liu, Z. F. Single gold-nanoparticle-enhanced raman scattering of individual single-walled carbon nanotubes via atomic force microscope manipulation. J. Phys. Chem. C 2008, 112, 7119–7123.

Nano Res. 2011, 4(5): 440–447

447

[12] Saito, N.; Haneda, H.; Sekiguchi, T.; Ohashi, N.; Sakaguchi, I.; Koumoto, K. Low-temperature fabrication of light-emitting zinc oxide micropatterns using self-assembled monolayers. Adv. Mater. 2002, 14, 418–421.

[13] Choi, D.; Choi, M. Y.; Choi, W. M.; Shin, H. J.; Seo, J. S.; Park, J.; Yoon, S. M.; Chae, S. J.; Lee, Y. H.; Kim, S. W.; Choi, J. Y.; Lee. S. Y.; Kim, J. M. Fully rollable transparent nanogenerators based on graphene electrodes. Adv. Mater. 2010, 22, 2187–2192.

[14] Wang, X.; Gao, Y.; Wei, Y.; Wang, Z. L. Output of an ultrasonic wave-driven nanogenerator in a confined tube. Nano Res. 2009, 2, 177–182.

[15] Kim, Y. J.; Lee, J. H.; Yi, G. C. Vertically aligned ZnO nanostructures grown on graphene layers. Appl. Phys. Lett. 2009, 95, 213101.

[16] Lee, J. M.; Pyun, Y. B.; Yi, J.; Choung, W.; Park, W. I. ZnO nanorod−graphene hybrid architectures for multifunctional conductors. J. Phys. Chem. C 2009, 113, 19134–19138.

[17] Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 1990, 41, 7892–7895.

[18] Choi, M. Y.; Choi, D.; Jin, M. J.; Kim, I.; Kim, S. H.; Choi, J. Y.; Lee, S. Y.; Kim, J. M.; Kim, S W. Mechanically powered transparent flexible charge-generating nanodevices with piezoelectric ZnO nanorods Adv. Mater. 2009, 21, 2185–2189.

[19] Choi, D.; Choi, M. Y.; Shin, H. J.; Yoon, S. M.; Seo, J. S.; Choi, J. Y.; Lee, S. Y.; Kim, J. M.; Kim, S. W. Nanoscale networked single-walled carbon-nanotube electrodes for transparent flexible nanogenerators. J. Phys. Chem. C 2010, 114, 1379–1384.

[20] Kim, K. K.; Lee, S. D.; Kim, H.; Park, J. C.; Lee, S. N.; Park, Y.; Park, S. J.; Kim, S. W. Enhanced light extraction efficiency of GaN-based light-emitting diodes with ZnO nanorod arrays grown using aqueous solution. Appl. Phys. Lett. 2009, 94, 071118.

[21] Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Making graphene visible. Appl. Phys. Lett. 2007, 91, 063124.

[22] Ni, Z. H.; Wang, H. M.; Kasmin, J.; Fan, H. M.; Yu, T.; Wu, Y. H.; Feng, Y. P.; Shen, Z. X. Graphene thickness determination using reflection and contrast spectroscopy. Nano Lett. 2007, 7, 2758–2763.

[23] Chae, S. J.; Gunes, F.; Kim, K. K.; Kim, E. S.; Han, G. H.; Kim, S. M.; Shin, H. J.; Yoon, S. M.; Choi, J. Y.; Park, M. H.; Yang, C. W.; Pribat, D.; Lee,Y. H. Synthesis of large-area graphene layers on poly-nickel substrate by chemical vapor deposition: Wrinkle formation. Adv. Mater. 2009, 21, 2328– 2333.

[24] Lee, S. D.; Kim, Y. S.; Yi, M. S.; Choi, J. Y.; Kim, S. W. Morphology control and electroluminescence of ZnO nanorod/GaN heterojunctions prepared using aqueous solution. J. Phys. Chem. C 2009, 113, 8954–8958.

[25] Xuan, Y.; Lin, H. C.; Ye, P. D.; Wilk, G. D. Capacitance- voltage studies on enhancement-mode InGaAs metal-oxide- semiconductor field-effect transistor using atomic-layer- deposited Al2O3 gate dielectric. Appl. Phys. Lett. 2006, 88, 263518.

[26] Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169–11186.

[27] Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane- wave basis set. Comput. Mater. Sci. 1996, 6, 15–19.

[28] Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758–1775.

[29] Nobis, T.; Kaidashev, E. M.; Rahm, A.; Lorenz, M.; Lenzner, J.; Grundmann, M. Spatially inhomogeneous impurity distribution in ZnO micropillars. Nano Lett. 2004, 4, 797–800.