Graphene/Carbon Nanotube Hybrid-Based Transparent 2D Optical Array

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

Un Jeong Kim , IL Ha Lee , Jung Jun Bae , Sangjin Lee , Gang Hee Han , Seung Jin Chae , Fethullah Günes , Jun Hee Choi , Chan Wook Baik , Sun Il Kim , * Jong Min Kim , * and Young Hee Lee*

Graphene/Carbon Nanotube Hybrid-Based Transparent 2D Optical Array

Holography is an ideal technology to realize 3D displays. Photo-refractive materials, in which the refractive index changes when exposed to light, have been used to realize active-type dynamic hologram images, but unfortunately they require high external voltages to operate. [ 1–5 ] To avoid such diffi culties, photo chromic materials such as azobenzene and diarylethene have been introduced but still have the drawbacks of long write/erase times. [ 6–11 ] Another approach was the use of liquid crystals to modulate the refractive index by controlling the external bias. However, the pixel size is limited to sizes larger than 5 μ m, which is a signifi cant drawback to realize holograms. [ 12 , 13 ]

An array of multiwalled carbon nanotube (MWCNT) pixels was fabricated to modulate the refractive index in a desired loca-tion. A strong local fi eld was generated by geometrically sharp carbon nanotubes that have high a fi eld enhancement factor, which eventually locally modulates the refractive index of the liquid crystal medium at low operation voltages. [ 14–16 ] Recently, carbon nanofi ber arrays were synthesized on TiN/quartz sub-strate with 28% optical loss. This is suitable for transmissive mode for optical devices, while refl ective mode is limited due to the possibility of interference between refl ective and diffractive lights. This method demonstrated relatively reasonable transmit-tance for this purpose, which could be used in a prototype design for holograms. Nevertheless, some of the light was absorbed in the cathode electrode due to the use of opaque metal. [ 17 ] On the other hand, graphene, a 2D hexagonal lattice of carbon atoms, is highly conductive with 97.5% transmittance per layer, [ 18 ] and has been used for several transparent and fl exible electrodes. [ 19–26 ]

The purpose of the research presented in this paper was to develop a transparent, active-type 2D optical array that oper-ates at low voltage. Instead of using opaque metals, a graphene electrode was used. An array of carbon nanotube pixels was

© 2011 WILEY-VCH Verlag GAdv. Mater. 2011, 23, 3809–3814

I. H. Lee , J. J. Bae , G. H. Han, S. J. Chae, F. Günes , Prof. Y. H. Lee BK21 Physics DivisionDepartment of Energy Scienceand Center for Nanotubes and Nanostructured CompositesSungkyunkwan Advanced Institute of NanotechnologySungkyunkwan UniversitySuwon 440-746, Korea E-mail: [email protected] U. J. Kim, S. Lee, J. H. Choi, C. W. Baik, Dr. S. I. Kim , J. M. Kim Frontier Research LabSamsung Advanced Institute of Technology (SAIT)P. O. Box 111, Suwon 440-600, Korea E-mail: [email protected]; [email protected]

DOI: 10.1002/adma.201101622

synthesized on patterned catalysts using chemical vapor depo-sition. CNTs were uniformly distributed on each pixel with a typical height of 1 ± 0.2 μ m, which was confi rmed by using an optical surface profi ler. The fabricated 2D grating array shows good diffraction effi ciency with an absolute transmittance of 97% at a relatively low operation voltage within ∼ 0.05 V/ μ m in a liquid crystal medium.

Figure 1 a is a schematic of the carbon nanotube/graphene electrode preparation. A graphene fi lm of 10 cm × 10 cm, grown on catalytic copper foil by chemical vapor deposition (CVD) with a methane/hydrogen gas mixture, was transferred by poly(methyl methacrylate) (PMMA) onto a quartz substrate. Bilayer graphene substrate was formed by repeating the process. The array of iron and aluminum catalysts was deposited in the desired position on the graphene by photolithography. Vertically aligned CNTs were grown by the CVD process described in the Experimental Section. Scanning electron microscopy (SEM) images show an array of CNT dots (Figure 1 b). Since the dot size is negligible due to the large separation distance between dots (15 μ m), the electrode is nearly transparent (inset) with a transmittance of 94%. No CNTs were grown on the graphene, whereas long CNT forests with a height of 1 μ m were formed on the catalyst dot. Graphene clearly shows a G-band near 1580 cm − 1 and a large G′/G intensity ratio with small D-band intensity in Raman spectroscopy (Figure 1 c). On the other hand, the sharp G-band intensity with a radial breathing mode (inset of panel d) near 200 cm − 1 indicates the formation of sin-glewalled carbon nanotubes (SWCNTs).

The height distribution of the SWCNT forests is important for obtaining uniform diffraction patterns. This distribution was measured using an optical surface profi ler. The top view is shown in Figure 2 a with a color profi le. SWCNTs were well synthesized in each pixel. The bottom panel demonstrates the uniformity of the SWCNT forests along the line shown in the upper panel. The top view and height distribution curve in Figure 2 b and 2c indicate that the average height is about 1 ± 0.2 μ m. The standard deviation was reduced with increasing the CNT forest heights, which was simply achieved by extending the growth time during CVD. Reduced height deviation is an important parameter in realizing reliable holography.

Graphene has been used for transparent electrodes due to its superior electrical conductivity and transmittance. Recently, large-area graphene was synthesized on a Cu sub-strate using the CVD technique and this can be, in gen-eral, simply adapted for a fl exible and transparent system to replace ITO. [ 27–29 ] However, in our approach, the graphene

mbH & Co. KGaA, Weinheim 3809wileyonlinelibrary.com

3810

www.advmat.dewww.MaterialsViews.com

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

CO

MM

UN

ICATI

ON

wileyonlinelibrary.com Adv. Mater. 2011, 23, 3809–3814

Figure 2 . 3D images obtained from optical surface profi ler. (a) Top view and the height profi le along the line in (a), (b) 3D image, and (c) histogram of CNT heights.

Figure 1 . (a) Fabrication schematics of graphene/VACNT hybrid structure on quartz. (b) SEM images of VACNTs graphene/quartz substrate. Digital photographic image of our optical element (inset). Raman spectroscopy of (c) synthesized graphene after transfer and (d) VACNTs. Inset shows the RBM of CNTs.

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

Figure 3 . (a) Transmittance of graphene fi lms as a function of the number of layers at a wavelength of 550 nm. Inset shows the transmittance of graphene as a function of the wavelength. (b) Sheet resistance changes with a variation in the number of layers before and after CVD process. (c) D/G ratio mapping by confocal Raman spectroscopy before and after the CVD process. (d) Raman spectroscopy with a wavelength of 532 nm before and after the CVD process.

electrode was placed in a high temperature chamber for CNT growth. Therefore, the tolerance of graphene electrodes to high temperatures during CNT growth must be guaranteed in advance. Figure 3 a shows the change in transmittance with an increasing number of graphene layers. The graphene layer was transferred onto the quartz substrate and the transmit-tance was measured by UV-vis-NIR spectrometry at 550 nm. The transmittance of a single graphene layer was ∼ 96%, as shown in the insets, slightly larger than the 97.5% of the pre-vious report. This difference was attributed to extra absorb-ance by a few layers formed locally in various places. [ 25 ] This value was further reduced to 85% for four layers of graphene. The sheet resistance of the pristine single-layer graphene was 680 Ω /sq and decreased to 276 Ω /sq for triple-layer graphene, as shown in Figure 3 b. A similar trend was observed even after CNT growth. Although the sheet resistance was slightly increased by about 20% in the case of three layers of graphene, the increase was limited to within 5% of the other cases. This tolerance is acceptable for electrodes. We fur-ther checked the uniformity of the fi lm by confocal Raman spectroscopy, as shown in Figure 3 c. The D/G intensity ratio was increased over a wide range of the graphene fi lm. This increase is attributed to defects generated by a hydrogen atmosphere and amorphous carbon deposition during the CVD process. Nonetheless, the uniformity was maintained similar to the pristine sample and, furthermore, no appreci-able etching such as large holes or cracks on the micrometer

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 3809–3814

scale was observed. The development of the D-band was clearly observed in the spectrum shown in Figure 3 d after the CVD process. The high transmittance, observed with four graphene layers without generating noticeable cracks, is suf-fi cient for realization of holography together with low sheet resistance. These phenomena may be attributed to the use of the remote PECVD system. The distance between plasma and substrate is variable and the direct infl uence of the plasma could be minimized, and thus mild growth conditions main-tain the desired transparency and resistance.

The diffraction pattern through the graphene/VACNTs hybrid structure is shown in Figure 4 as a function of the applied voltage from 0 ∼ 6 V. A He-Ne Laser with a 633 nm wavelength was used to generate diffraction patterns from the graphene/VACNT hybrid structure on a quartz sub-strate, as shown in Figure 4 a. The liquid crystal director was aligned homogeneously parallel to the upper electrode, whose anchoring was invoked by the rubbing polyimide, and liquid crystal director was aligned randomly in the graphene plane. Thus, the light is transmitted partially from graphene region. On the other hand, CNTs absorb light and emit light again like an antenna, which act as a slit to generate a spherical wave. A series of light transmitted through two-dimensional slits behaves like two-dimensional Young’s slit, creating a planar interference pattern. Liquid crystal molecules near the CNTs can be aligned parallel to the long axis of CNT, [ 30 , 31 ] and in particular those located at the tip of CNTs are aligned

3811bH & Co. KGaA, Weinheim wileyonlinelibrary.com

3812

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

Figure 4 . Schematic of (a) the measurement for diffraction pattern and (b) the structure of the fabricated optical device before measurement. Liquid crystals are aligned by electric fi elds with an applied bias between two electrodes (20 μ m). Diffraction pattern of digital photographic images with dif-ferent bias at (c) 0 V, (d) 2 V, and (e) over 6 V.

more strongly parallel to the fi eld direction, where the fi eld is strongly enhanced by the sharp CNT tip. [ 32 ] The path dif-ference of light from slits can be modulated by the degree of alignment of liquid crystal molecules in each slit, which can be varied by external electric fi eld. Each pixel thus acts as a circular slit, and therefore a 2D diffraction pattern from peri-odically arranged 2D CNT pixels (15 μ m space in the current study) was observed, as shown in Figure 4 c. With increasing voltage to 2 V, a strong local fi eld enhanced by the CNT tips changes the liquid crystal director nearby to modify the local refractive index. Therefore, Δ n in the local area increased the change in light intensity of the diffraction pattern, as shown in Figure 4 d. [ 14–17 ] At a high voltage of 6 V, all the liquid crystal directors were aligned along the fi eld direction, therefore Δ n ∼ 0, invoking no diffraction pattern changes and thus reducing light intensity (Figure 4 e). The formation of diffraction pattern shown in the fi gure was retained independent of the direction of liquid crystal molecules (or applied voltage) and observed even in air cell.

Figure 5 shows the detailed light intensity changes at the 1 th order spot as a function of voltage for different tip morphologies. With increasing voltage, more liquid crystal molecules are aligned along the field, invoking larger Δ n near the tip. This increased the intensity of the 1 th order until a maximum value was reached. At very high voltage, the alignment of liquid crystal molecules no longer occurs,

© 2011 WILEY-VCH Verlag Gwileyonlinelibrary.com

and therefore Δ n decreases, reducing the intensity of the dif-fracted 1th order light. A similar trend was observed inde-pendent of the tip morphology and type of CNTs. The peak voltage was reduced when the SWCNTs were aligned better (pillar 2) due to a more efficient alignment of liquid crystal molecules near the CNTs. The maximum diffraction inten-sity was observed around 2 ∼ 3 V. This is modulated by the region of distorted liquid crystal molecules near the CNTs. A higher peak voltage was required for MWCNTs due to poor alignment of MWCNTs or a smaller field enhancement factor. [ 30 ] It is further noted that the degree of alignment of the CNTs plays an important role in improving the diffrac-tion light intensity. For instance, Pillar 2, which has a better alignment than Pillar 1, has four times stronger intensity (Figure 5 e).

In summary, we fabricated transparent optical elements using graphene/CNT hybrid structure. Nearly 1 μ m long and 2 μ m dot sizes of VACNTs with 15 μ m spacing between tubes were grown on transparent graphene/quartz substrate by PECVD. Transparency and conductivity were maintained under harsh environments such as a hydrocarbon gas atmos-phere at high temperatures. In order to prevent the reduction of the diffracted light intensity, it is important to keep the CNTs highly aligned. Our current approach of the transparent and fl exible optical element could be benefi cial for future 3D holography.

mbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 3809–3814

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

Figure 5 . SEM images of (a) SWCNT Pillar 1, (b) SWCNT Pillar 2, and (c) MWCNT pillar. (d) Normalized diffraction intensity of three different CNT pillars which depend on the applied bias. (e) Absolute intensity difference between SWCNT Pillar 1 and SWCNT Pillar 2.

Experimental Section Graphene/VACNT hybrid structure : Graphene was synthesized on a Cu

foil (75 μ m) by thermal chemical vapor deposition (CVD) and transferred to the quartz substrate. The CVD chamber was heated to 1000 ° C and CH 4 (5 sccm), Ar (1000 sccm), and H 2 (200 sccm) gases were introduced to the chamber for 2 min. The details have been published elsewhere. [ 20 , 28 ] PMMA was spin-coated onto a graphene/Cu substrate and the Cu fi lm was removed by submerging the fi lm into Cu etchant (CE-100, TRANSENE COMPANY) and rinsing several times until no Cu residual was noticed. The graphene/PMMA fi lm was then transferred to a quartz substrate. [ 20 , 28 ] This process was repeated to obtain several layers of graphene. Conventional photolithography was used to make dot arrays with a ∼ 1 μ m dot size and ∼ 15 μ m spacing between dots. 10 nm Al and 1 nm Fe layers were deposited on this as a catalyst for CNT growth. VACNTs were synthesized by remote plasma-enhanced CVD with Ar (200 sccm), H 2 (200 sccm), and C 2 H 4 (75 sccm) gases at 750 ° C for 5 min.

Optical cell assembly : The graphene / VACNTs hybrid structure was covered with the top ITO glass electrode separated by a 20 μ m spacer. ITO glass was coated with polyimide and rubbed for horizontal alignment of the liquid crystal. A nematic liquid crystal with positive dielectric anisotropy (Merck, ZLI 4792) was injected into the cell.

Measurements : Field-emission scanning electron microscopy (FESEM, JSM 7000F, JEOL) was used to observe the morphology of graphene/VACNTs hybrid structure. The overall height of VACNTs arrays was analyzed using an optical surface profi ler (NewView 7300, Zygo Corporation). Raman spectroscopy (Renishaw, RM-1000 Invia) with an excitation energy of 2.41 eV (514 nm, Ar-ion laser) was used for the characterization of graphene and VACNTs. Finally, confocal Raman spectroscopy (Witec, CRM-200) with a 532 nm wavelength was also

© 2011 WILEY-VCH Verlag GAdv. Mater. 2011, 23, 3809–3814

used to compare the graphene surface before and after the CVD process by mapping the D/G ratio. Sheet resistance was measured by a four-point method (Keithley 2000 multimeter) at room temperature. Optical properties of graphene fi lms were analyzed by Uv-vis-NIR absorption spectroscopy (Varian, Cary 5000).

Acknowledgements U.J.K. and I.H.L. contributed equally to this work. This work was supported by the WCU (World Class University) program through the NRF funded by the MEST (R31-2008-10029), the Korean government (MEST) (No. 2010-0029653), the Industrial Technology Development Program (10031734) of the Ministry of Knowledge Economy (MKE), and the IRDP of NRF (2010-00429) through a grant provided by MEST in 2010 in Korea.

Received: April 29, 2011Published online: July 19, 2011

[ 1 ] O. Ostroverkhova , W. E. Moerner , Chem. Rev. 2004 , 104 , 3267 . [ 2 ] S. Tay , P. -A. Blanche , R. Voorakaranam , A. V. Tunc , W. Lin ,

S. Rokutanda , T. Gu , D. Flores , P. Wang , G. Li , P. St Hilaire , J. Thomas , R. A. Norwood , M. Yamamoto , N. Peyghambarian , Nature 2008 , 451 , 694 .

[ 3 ] P.-A. Blanche , A. Bablumian , R. Voorakaranam , C. Christenson , W. Lin , T. Gu , D. Flores , P. Wang , W.-Y. Hsieh , M. Kathaperumal , B. Rachwal , O. Siddiqui , J. Thomas , R. A. Norwood , M. Yamamoto , N. Peyghambarian , Nature 2010 , 468 , 80 .

3813mbH & Co. KGaA, Weinheim wileyonlinelibrary.com

3814

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

[ 5 ] S. Lee , Y.-C. Jeong , Y. Heo , S. I. Kim , Y.-S. Choi , J.-K. Park , J. Mater. Chem. 2009 , 19 , 1105 .

[ 6 ] A. Saishoji , D. Sato , A. Shishido , T. Ikeda , Langmuir 2007 , 23 , 320 . [ 7 ] F. You , M. Y. Paik , M. Hackel , L. Kador , D. Kropp , H.-W. Schmidt ,

C. K. Ober , Adv. Funct. Mater. 2006 , 16 , 1577 . [ 8 ] F. Gallego-Gomez , F. del Monte , K. Meerholz , Nat. Mater. 2008 , 7 ,

490 . [ 9 ] S. Pu , T. Yang , B. Yao , Y. Wang , M. Lei , J. Xu , Mater. Lett. 2007 , 61 ,

855 . [ 10 ] J. Minabe , S. Yasuda , K. Kawano , T. Maruyama , H. Yamada , Jpn. J.

Appl. Phys. 2003 , 42 , L426 . [ 11 ] C. Erben , E. P. Boden , K. L. Longley , X. Shi , B. L. Lawrence , Adv.

Func. Mater. 2007 , 17 , 2659 . [ 12 ] H. Dai , K. Xu , Y. Liu , X. Wang , J. Liu , Opt. Comm. 2004 , 238 , 269 . [ 13 ] P. J. M. Vanbrabant , J. Beeckman , K. Neyts , E. Willman ,

F. A. Fernandez , J. Appl. Phys. 2010 , 108 , 08314-1 . [ 14 ] T. D. Wilkinson , X. Wang , K. B. K. Teo , W. I. Milne , Adv. Mater. 2008 ,

20 , 363 . [ 15 ] R. Rajasekharan-Unnithan , H. Butt , T. D. Wilkinson , Opt. Lett. 2009 ,

34 , 1237 . [ 16 ] R. Rajasekharan , C. Bay , Q. Dai , J. Freeman , T. D. Wilkinson , Appl.

Phys. Lett. 2010 , 96 , 233108-1 . [ 17 ] Q. Dai , R. Rajasekharan , H. Butt , K. Won , X. Wang , T. D. Wilkinson ,

G. Amaragtunga , Nanotechnology 2011 , 22 , 115201 . [ 18 ] R. R. Nair , P. Blake , A. N. Grigorenko , K. S. Novoselov , T. J. Booth ,

T. Stauber , N. M. R. Peres , A. K. Geim , Science 2008 , 320 , 1308 . [ 19 ] K. S. Kim , Y. Zhao , H. Jang , S. Y. Lee , J. M. Kim , K. S. Kim , J.-H. Ahn ,

P. Kim , J.-Y. Choi , B. H. Hong , Nature 2009 , 457 , 706 .

© 2011 WILEY-VCH Verlag wileyonlinelibrary.com

[ 20 ] F. Günes , G. H. Han , K. K. Kim , E. S. Kim , S. J. Chae , M. H. Park , H.-K. Jeong , S. C. Lim , Y. H. Lee , NANO 2009 , 4 , 83 .

[ 21 ] X. Wang , L. Zhi , K. Mullen , Nano Lett. 2008 , 8 , 323 . [ 22 ] J. Wu , M. Agrawal , H. A. Becerril , Z. Bao , Z. Liu , Y. Chen ,

P. Peumans , ACS Nano 2010 , 4 , 43 . [ 23 ] D. Choi , M. -Y. Choi , W. M. Choi , H. -J. Shin , H.-K. Park , J. -S. Seo ,

J. Park , S. -M. Yoon , S. J. Chae , Y. H. Lee , S. -W. Kim , J. -Y. Choi , S. Y. Lee , J. M. Kim , Adv. Mater. 2010 , 22 , 2187 .

[ 24 ] C.-A. Di , D. Wei , G. Yu , Y. Liu , Y. Guo , D. Zhu , Adv. Mater. 2008 , 20 , 3289 .

[ 25 ] F. Günes , H.-J. Shin , C. Biswas , G. H. Han , E. S. Kim , S. J. Chae , J.-Y. Choi , Y. H. Lee , ACS Nano 2010 , 4 , 4595 .

[ 26 ] P. Blake , P. D. Brimicombe , R. R. Nair , T. J. Booth , D. Jiang , F. Schedin , L. A. Ponomarenko , S. V. Morozov , H. F. Gleeson , E. W. Hill , A. K. Geim , K. S. Novoselov , Nano Lett. 2008 , 8 , 1704 .

[ 27 ] X. Li , W. Cai , J. An , S. Kim , J. Nah , D. Yang , R. Piner , A. Velamakanni , I. Jung , E. Tutuc , S. K. Banerjee , L. Colombo , R. S. Ruoff , Science 2009 , 324 , 1312 .

[ 28 ] G. H. Han , H.-J. Shin , E. S. Kim , S. J. Chae , J.-Y. Choi , Y. H. Lee , NANO 2011 , 6 , 59 .

[ 29 ] A. Reina , X. Jia , J. Ho , D. Nezich , H. Son , V. Bulovic , M. S. Dresselhaus , J. Kong , Nano Lett. 2009 , 9 , 30 .

[ 30 ] S. J. Jeong , K. A. Park , S. H. Jeong , H. J. Jeong , K. H. An , C. W. Nah , D. Pribat , S. H. Lee , Y. H. Lee , Nano Lett. 2007 , 7 , 2178 .

[ 31 ] S. Y. Jeon , K. A. Park , I.-S. Baik , S. J. Jeong , S. H. Jeong , K. H. An , S. H. Lee , Y. H. Lee , NANO 2007 , 2 , 41 .

[ 32 ] H. J. Jeong , H. K. Choi , G. Y. Kim , Y. I. Song , Y. Tong , S. C. Lim , Y. H. Lee , Carbon 2006 , 44 , 2689 .

GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 3809–3814