Making Patterns on Graphene

A

www.advmat.dewww.MaterialsViews.com

RESM

Gsgmarpt

aking Patterns on Graphene

EARCH

By Yong Zhou * and Kian Ping Loh *

NEW

Sraphene-based nanostructures are considered as promising alternatives to ilicon-based mesostructures in future electronic nanodevices. The litho-raphical patterning of graphene, which are essential steps in any form of icroelectronic processing, present interesting challenges because of the

tomic layer thickness of graphene. Mesoscopic devices based on graphene equire high spatial resolution patterning that will induce as little damage as ossible. This research news highlights and evaluates recent developments in he nanostructuring and patterning of graphene.

1. Introduction

Graphene is a perfect two-dimensional (2D) carbon sheet that forms the basic building block of graphite, fullerene and carbon nanotubes. Since its isolation in 2004, [ 1 ] it provides the most powerful proof that free-standing atomically thin materials can exist at ambient conditions. Graphene has attracted signifi cant attention because of its unique electronic properties, such as the charge carriers mimicking massless Dirac fermions, elec-tron-hole symmetry near the charge neutrality point, weak spin-orbit coupling, high mobility [ 2 ] and saturation velocity for both electrons and holes, [ 3 ] room-temperature quantum Hall effect, [ 4 ] and good optical transparency. [ 5 ] Graphene provides an alter-native high mobility electronic platform to silicon based mes-ostructures. In terms of vertical scaling, graphene is literally scalable to one atomic layer, however considerable challenges exist in the lithographical patterning of graphene because of its thinness and the sensitivity of its electrical properties to con-tamination by residual photoresists. [ 6 ]

There are a number of works on the preparation methods of graphene, these include the micromechanical cleavage of highly oriented pyrolytic graphite (HOPG), [ 7 ] epitaxial growth on sil-icon carbide, [ 8 ] chemical exfoliation and reduction of graphene oxide [ 9 , 10 ] as well as the latest breakthroughs in chemical vapor deposition of large area graphene. [ 11 , 12 ] Graphene derived from

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimdv. Mater. 2010, 22, 3615–3620

[∗] Prof. Y. Zhou Ecomaterials and Renewable Energy Research Center (ERERC)School of Physcis, National Laboratory of Solid State MicrostructuresNanjing University22 Hankou Road, Nanjing, Jiangsu 210093 (P. R. China) E-mail: [email protected] Prof. K. P. Loh Department of Chemistry Faculty of Science National University of Singapore (NUS) 3 Science Drive 3, Singapore 117543 (Singapore) E-mail: [email protected]

DOI: 10.1002/adma.201000436

chemical processing are usually irregular in shapes and are not suitable for inte-grated circuit manufacturing. The most promising approach today for large scale integration has to be accorded to roll-to-roll graphene fi lms produced by chemical vapor deposition. After the graphene layer is transferred onto a substrate, it will be advantageous to develop a high throughput method for the patterning of graphene to fabricate dimensional nanostructures such as graphene nanoribbons. For multilayer

graphene or graphene with non-uniform thickness, a method which allows the “shearing” of extraneous layers down to the desired thickness will be very useful in ensuring wide area uniformity. To address these, methods to transfer, pattern and nanostructure graphene layers must be developed. Recently, cre-ative methods to micro/nanostructure graphene are emerging so it is timely to review the latest developments in this fi eld.

2. Making Patterns on Graphene

Existing methods of patterning graphene include mask lithog-raphy, transfer printing, and direct-write process. The utilization of masks as sacrifi cial materials is a convenient way to pattern graphene on a large scale. [ 13–15 ] Using polystyrene (PS) nano-sphere as a mask in combination with O 2 reactive ion etching (RIE) was reported to produce periodically ordered graphene nanodisk arrays. [ 13 ] A monolayer of highly ordered PS spheres was self-assembled on water surface, [ 16 ] this monolayer was then lifted off and transferred onto graphene sheet on SiO 2 /Si ( Figure 1a and 1b ). O 2 RIE was subsequently carried out to etch a portion of the graphene sheets that was not protected by the PS nanospheres (Figure 1c ). The PS spheres were then removed by sonication in chloroform, resulting in periodic graphene nanodisk arrays (Figure 1d and 1e ). The dimension of graphene nanodisks can be effectively tuned by varying the size of the PS spheres, which can be scaled down to tens of nanometers. Such graphene nanodisks may be used as microelectrode array in electrochemistry. Aluminum [ 14 ] and hydrogen silsesquioxane [ 15 ] masks have also been used for the creation of graphene pat-terns. Although mask lithography is suitable for large-scale fab-rication of a variety of patterns; it is limited by under-etching and contamination from the contacting masks. The edges gen-erated by etching through shadow or resist masks can produce disordered edges that affect the properties of graphene ribbons.

Advances in transfer technology must keep pace with the development of large area graphene fi lms, since the transfer of graphene fi lm onto different types of substrates will be necessary for wide ranging applications. Transfer printing was a

3615

3616


RES

EARCH N

EWS

Yong Zhou studied chemistry and physics at the University of Science and Technology of China (USTC), received his PhD degree there in 2000. After working in Kyoto University in 2000–01, Max Planck Institute of Colloids and Interfaces in 2002–03, the National Institute of Materials Science (NIMS, Japan) in 2003–04, National Institute

of Advanced Industrial Science and Technology (AIST, Japan) in 2004–08, and National University of Singapore (NUS) in 2008–09, he joined as a full professor in the Eco-materials and Renewable Energy Research Center (ERERC), School of Physyics, National Laboratory of Solid State Microstructures, Nanjing University, China. His research now focuses on clean energy materials, graphene-based devices, and bio-inspired nanomaterials.

Professor Kian Ping Loh has worked in the National University of Singapore for more than ten years and leads the effort in functional carbon materials research. He obtained his Ph.D. from the Physical and Theoretical Chemistry Laboratory in Oxford in 1997 and con-tinued his postdoctoral work in National Institute for

Materials in Japan between 1997–98. His research interest is focused on advanced functional carbon materials, and includes the synthesis and modifi cation of graphene. Breakthroughs in his research include the fabrication of organic solar cell on transparent and conducting, large area graphene electrodes, and the fi rst demonstration of wide band tuneable mode locking on graphene. He has won the University Young Researcher Award in 2008 and the Outstanding Chemist Award in 2009.

Figure 1 . Schematic illustration for graphene nanodisk arrays fabrication. a) The graphene sheets were transferred to SiO 2 /Si substrate. b) The monolayer PS spheres form an ordered array on the SiO 2 /Si substrate coated with graphene. c) A portion of graphene sheets unprotected by the PS spheres was etched with O 2 RIE. d) Removal of PS spheres by soni-cation in chloroform. e) SEM image of graphene disk arrays. (e) Repro-duced with permission. [ 13 ] Copyright 2009, American Chemical Society.

soft contact method well-suited for the generation of graphene pattern. Graphene-coated glass substrates was contacted with a patterned poly(dimethylsiloxane) (PDMS) stamp, i.e. “inking” the stamp ( Figure 2a and 2b ). [ 17 ] The inked stamp is contacted with a Si/SiO 2 substrate. As the binding energy of PDMS for graphene is weaker than the substrate interface, graphene is readily transferred from the PDMS to Si/SiO 2 substrates at room temperature (Figure 2c ). The stamp is fi nally peeled

© 2010 WILEY-VCH Verlag G

from the substrate, leaving behind the rectangular pattern of graphene (Figure 2d ). Combination of the stamp transfer process with a layer-by-layer (LBL) method affords a strategy for the patterning of conductive multilayered polymer/exfoliated graphite hybrid fi lms. [ 18 ] Liang et al. exploited PDMS stamp-based cutting and exfoliation to transfer-print graphene fea-tures precisely onto a substrate. [ 19 ] The transfer mechanism is based on the different strengths of non-covalent adhesion at the pre-patterned stamp-graphene and graphene-substrate inter-faces. A stamp with protrusions was pressed onto the graphite substrate. Since the bonding between the stamp and graphene is stronger than between the graphene layers, the stamp cuts and attaches to a layer of graphene, the separation of the stamp

mbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 3615–3620


RES

EARCH N

EWS

Figure 2 . Diagram (left) and optical microscope images (right) depicting the PDMS transfer process. It begins by (a) depositing materials on a glass substrate and (b) carefully “inking” the pre-patterned PDMS stamp. c) The inked stamp contacts with a heated Si/SiO 2 substrate and (d) peeled away to reveal deposited materials. Reproduced with permission. [ 17 ]

Figure 3 . Atomistic model for the stamping of graphene. a) Pre-patterned graphite stamp with SiO 2 /Si. b) Stamping of grapheme. c) AFM image of a thin stamped square. (c) reproduced with permission. [ 20 ]

from the graphite exfoliates the graphene sheet. The isolated graphene can be transferred from the stamp onto the channel region of the device with potential nanoscale accuracy. Li et al. directly contacted the pre-patterned graphite stamp with SiO 2 /Si ( Figure 3a ). [ 20 ] There is evidence of strong adhesion between graphene and SiO 2 , [ 10 g] and this adhesive interaction screens the van der Waals bonding between graphite layers adjacent to the graphite/SiO 2 interface. Withdrawal of the graphite from the substrate results in the cleavage of graphite layers at the graphite/SiO 2 interface (Figure 3b and 3c ). Implementation of a voltage between graphite and Si generates an electrostatic screening force among the graphite layers which enhances the exfoliation process. [ 21 ] There is still much room in terms of improving the uniformity and reproducibility of transfer-printed fi lm for large-scale fabrication.

Charged molecular templates had been employed to transfer and immobilize single-layer graphene onto predefi ned areas of substrate surfaces. [ 22 ] The molecular templates were generated using the microcontact printing of 11-amino-1-undecanethiol self-assembled monolayer on mica-peeled Au substrates. [ 23 ] These templates were then immersed into aqueous graphene oxide (GO) dispersions so that the electrostatic interactions between the negatively charged oxygen-containing functional groups on GO sheets and the positively protonated amine groups of 11-amino-1-undecanethiol transfer the GO sheets onto the patterned functionalized surfaces. The adsorption

© 2010 WILEY-VCH Verlag GmAdv. Mater. 2010, 22, 3615–3620

process can be optimized by tuning various parameters such as pH, immersion time, surface passivation, and the concentra-tion of the GO dispersion. The immobilized GO sheets could be transformed to graphene sheets by thermal annealing.

Few-layer or monolayer graphene can be deposited by the chemical vapor deposition (CVD) of hydrocarbons on (e.g. Ni, Cu and Ag) metallic substrates. [ 11 , 24 ] Hong et al. used the CVD process to grow high-quality, few-layer graphene fi lms on nickel-coated SiO 2 /Si substrates. [ 22 ] By using the pre-patterned nickel substrate, various sizes and shapes of graphene fi lm can be obtained. The average number of graphene layers, the domain size and the substrate coverage can be controlled by changing the nickel thickness and growth time during the growth process. Etching nickel substrate layers with FeCl 3 solu-tion allows one to readily transfer patterned graphene fi lms to an arbitrary substrate for device applications.

3617bH & Co. KGaA, Weinheim

3618


RES

EARCH N

EWS

Figure 4 . a) Schematic illustration of the creation of micro-features on GO nanosheets with an optical microscope-focused laser beam. 1: Quartz substrate; 2: Multilayer fi lm of GO; 3: Laser source; 4: GO channel pattern; 5: Graphene channel pattern; 6: Graphene pattern in the GO fi lm. For simplicity of the illustration, the hydroxyl, ether, and carboxylic groups of GO were omitted, and use of yellow color represents GO and blue color for graphene. b) Optical microscope image of the GO channel. The bright region corresponds to cut area, and dim regions to uncut one. c) SEM image of the GO channel and inset for 3D AFM image. (b) and (c) reproduced with permission. [ 26 ]

Strong adhesion between gold and carbon surface can also be exploited for transfering pre-patterned graphene layers. [ 25 ] Gold fi lm was deposited onto the patterned HOPG disk surface, and a thermal release tape was used to peel off the gold fi lm together with the graphene patterns. The gold fi lm attached with graphene patterns was then pressed onto any preheated target substrate. The heat-transfer process will render the graphene tightly attached to the substrate. The thermal tape was subsequently released and removed from the gold fi lm. The gold fi lm was then etched using etchant solution (KI–I 2 complex), leaving only graphene patterns on the target sub-strates over large areas.

Conventional lithographic techniques employ photoresists for the defi nition of patterns on substrates. Application of photoresist is undesirable because of the presence of residual polymers that contaminate the graphene surface, these interfere with subsequent metalliza-tion steps as well as introduce impurity doping and electrical noise. Non-contact patterning techniques based on direct-write process has great potential to emerge as a high through-put, parallel processing technique for the patterning of graphene. The advantage of this technique is its reliability, amenability, upwards scalability and low cost. Most importantly, the direct-write process reduces the number of masking or pho-toresist steps needed when designing an integrated circuit based on all-carbon
electronics. Recently, we developed a focused laser beam tech-nique to construct an extended area of micropatterned GO and reduced GO multilayers on quartz substrates in a fast and con-trolled manner. [ 26 ] Electrostatic LBL assembly techniques were employed to grow multilayers of GO fi lm on a quartz substrate with polyethylenimine as a linker. The fi lm was placed in the focused laser-beam system (Process 1 of Figure 4a , abbre-viated as P1). When the focused laser beam was incident on the multilayer GO fi lm, the irradiated area absorbed the laser energy, and the energy was rapidly converted into local heat. The intense heating raised the temperature of the irradiated area above 500 ° C in air and resulted in localized oxidative burning of GO to volatile gases such as CO or CO 2 . By moving the computer-controlled sample stage in a programmable step with respect to the focused laser beam, patterns with tunable width and length could be directly written (Figure 4a -P2). The patterned GO can be subsequently reduced to graphene by exposure to hydrazine gas or thermal annealing at high tem-peratures (Figure 4a -P3). When the focused laser-beam cutting of the GO fi lm was carried out with the sample housed in an inert environment (N 2 gas), we found that oxidative burning did
© 2010 WILEY-VCH Verlag Gm

not occur and there was no cutting. Instead, the GO was deoxy-genated or thermally reduced to graphene in this case. In other words, the direct writing of conductive G pattern on GO fi lms, where the graphene domains were isolated by insulating GO matrix, was achieved (Figure 4a -P1’). Our approach presents very unique advantages in device fabrication which cannot be achieved by any other techniques so far. The results show that thick fi lms ( > 5 layers) can absorb suffi cient thermal energy to reach the evaporation temperature of oxidized carbonaceous materials, as opposed to thin fi lms ( < 5 layers). It appears that the specifi c heat of the system scales with the thickness of the GO layers in an inverse manner, i.e. the thicker layers are easier to ablate than the thinner ones, because of the increased optical absorption length for thicker fi lms and the heat contribution of fl exural modes. As a result, the laser etching is self-terminating and a 6-layer fi lm is uniformly cut to a 3-layer fi lm. It can be anticipated that by optimizing parameters like the thermal con-duction properties and temperature of the substrate as well as the energy of the laser, a greater degree of control in terms of trimming the layers to the desired thickness may be possible. For example, it should be possible to apply laser ablation to

bH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 3615–3620


RES

EARCH N

EWS

Figure 5 . a) Large area growth of graphene by CVD. b) Transfer of graphene onto SiO 2 /Si sub-strate with transfer printing technique. c) Laser cut for rough pattern. d) Helium ion ablation for fi ne pattern. Top-right: GNR image produced with helium ion ablation. Machining series for the determination of helium ion dose needed to create nano-ribbons. Dose indicated by each sample, in units of 1018 ions cm − 2 . Right-top images reproduced with permission. [ 30 ] Copyright 2009, Carl Zeiss NTS GmbH.

shear multilayers graphene into monolayers by controlling the laser energy, with the abla-tion threshold energy inversely proportional to the layer thickness in a non-linear way.

The laser shearing method is useful for the high throughput microstruturing of graphene fi lms in the lateral scale, as well as for ver-tical scaling of multilayer fi lms to monolayer fi lm. However, the ultimate lateral resolution attainable is limited by the wavelength of the laser. For the nanostructuring of graphene fi lms, techniques with higher resolution must be applied. This is particularly relevant to the fabrication of graphene nanoribbons. A two-dimensional graphene sheet is a semimetal with no bandgap, making transistor with a reasonable turn-on ratio from such sheet is inherently problematic. Nanostructuring the graphene sheet into one-dimensional graphene nanoribbons (GNRs) with narrow widths (sub-10 nm) and atomically smooth edges creates effective quantum confi nement which opens up a fi nite band gap; on-off ratios of about 10 7 has been demonstrated on GNR transistor at room temperature. [ 27 ] Focus Ion Beam etching has been applied to
make GNR, but the technique requires the transfer of graphene fl akes onto TEM grids, which is not suitable for larger scale fab-rication of devices. The resolution of resist-based lithography is currently limited to a few tens of nanometers in terms of the width of the GNR. One drawback of resist-based lithography is the inevitable deposition of residues on the graphene, which will cause it to ripple.
To date, the nanostructuring of graphene to make GNR is most convincingly demonstrated by helium ion microscopy. [ 28 ] Helium ion microscopy is a recently developed high-resolution imaging technology useful for the imaging of carbon nanostructures. Helium ion microscopy is based on the fi eld ionization of helium ions using a cryogenically cooled tungsten tip, which is truncated by a trimer of atoms where imaging from a single atom is possible by precision alignment. Being a charged ion beam instrument, it is also possible to perform milling and sputtering, similar to a gallium focused ion beam (FIB) system. Helium ion lithography (HIL) is more adapted to the low rate milling and sputtering of soft and fragile materials compared to FIB. [ 29 ] Due to the very high source brightness and the short de Broglie wavelength of helium ion, which is 100 times smaller than the corresponding electron, it is possible to focus the ion beam to a much smaller probe size than that attainable by a scanning electron microscope. This gives the helium ion beam an ultimate resolution of 0.5 nm or better, making it an attractive and important tool for creating GNRs. Recently, Daniel Pickard of the National University of Sin-gapore successfully fabricated GNR on suspended graphene using helium ion beam. GNRs with smooth edges and widths in the sub-10 nm range have been attained (Right-top of Figure 5 ). [ 30 ] In principle, if the orientation of the graphene is known, GNR with either the “armchair” or “zig zag” confi guration can be produced.

For process integration with semiconductor manufacturing, an automated pattern generation system can be used for the

© 2010 WILEY-VCH Verlag GmbAdv. Mater. 2010, 22, 3615–3620

controlled etching of wafer scale graphene. Figure 5 illus-trates a possible process fl ow which can be used for the micro- and nanostructuring of graphene. CVD grown graphene is transferred onto the substrate by the transfer printing tech-nique, and then sculptured by laser writing to create micron-sized domains. These domains can be further cut into GNR with sharp edge profi le by fi ne patterning with the helium ion beam.

3. Conclusions and Outlook

The patterning of graphene is necessary for both electrical measurements as well as the fabrication of dimensional graphene devices. Resist-based lithography is a convenient way to defi ne graphene features and patterns for devices but suf-fers from multiple issues in the high throughput processing of graphene devices due to surface contamination issues. The maskless, contact-free, direct “writing” of patterns in graphene using laser or heium ion beam allows simultaneous lateral and vertical scaling. These technologies allow the fabrication of microscale and nanoscale features on graphene readily and pave the way for the integration of all-carbon electronics in the semiconductor industries.

Acknowledgements The authors acknowledge the support of the NRF-CRP grant “Graphene Related Materials and Devices R-143-000-360-281” for the support of this project.

Received: April 2, 2010 Published online: June 8, 2010

3619H & Co. KGaA, Weinheim

3620


RES

EARCH N

EWS

[ 1

[ 1

[ 1

[ 1

[ 1 [ 1

[ 1 ] K. S. Novoselov , A. K. Geim , S. V. Morozov , D. Jiang , Y. Zhang , S. V. Dubbonos , I. V. Grigorieva , A. A. Firsov , Science 2004 , 306 , 666 .

[ 2 ] A. K. Geim , K. S. Novoselov , Nat. Mater. 2007 , 6 , 183 . [ 3 ] H. B. Heersche , P. Jarillo-Herrero , J. B. Oostinga , L. M. K. Vander-

sypen , A. Morpurgo , Nature 2007 , 446 , 56 . [ 4 ] K. S. Novoselov , A. K. Geim , S. V. Morozov , D. Jiang , M. I. Kat-

snelson , I. V. Grigorieva , S. V. Dubonos , A. A. Firsov , Nature 2005 , 438 , 197 .

[ 5 ] X. Wang , L. Zhi , K. Müllen , Nano Lett. 2008 , 8 , 323 . [ 6 ] K. S. Novoselov , Z. Jiang , Y. Zhang , S. V. Morozov , H. L. Stormer , U.

Zeitler , G. S. Boebinger , P. Kim , A. K. Geim , Science 2007 , 315 , 1379 . [ 7 ] A. K. Geim , Science 2009 , 324 , 1530 . [ 8 ] T. Ohta , A. Bostwick , T. Seyller , K. Horn , E. Rotenberg , Science 2006 ,

313 , 951 . [ 9 ] a) Y. Zhou , Q. L. Bao , L. Tang , Y. Zhong , K. P. Loh , Chem. Mater. 2009 ,

21 , 2950 ; b) H. P. Cong , J. J. He , Y. Lu , S. H. Yu , Small 2010 , 6 , 169 . 0 ] a) S. Park , R. S. Ruoff , Nat. Nanotechnol. 2009 , 4 , 217 ; b) P. Avouris ,

Z. H. Chen , V. Perebeinos , Nat. Nanotechnol. 2007 , 2 , 605 ; c) D. Li , R. B. Kaner , Science 2008 , 320 , 1170 ; d) R. M. Westervelt , Science 2008 , 320 , 324 ; e) R. Van Noorden , Nature 2006 , 442 , 228 .

1 ] K. S. Kim , Y. Zhao , H. Jang , S. Y. Lee , J. M. Kim , Kwang , S. Kim , J. H. Ahn , P. Kim , J. Y. Choi , B. H. Hong , Nature 2009 , 457 , 706 .

2 ] 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 .

3 ] C. X. Cong , T. Yu , Z. H. Ni , L. Liu , Z. X. Chen , W. Huang , J. Phys. Chem. C 2009 , 113 , 6529 .

4 ] S. Pang , H. N. Tsao , X. Feng , K. Müllen , Adv. Mater. 2009 , 21 , 3488 . 5 ] a) M. Y. Han , B. Özyilmaz , Y. Zhang , P. Kim , Phys. Rev. Lett. 2007 ,

98 , 206805(4) ; b) Z. Chen , Y. M. Lin , M. J. Rooks , P. Avouris , Physica E 2007 , 40 , 228 .

© 2010 WILEY-VCH Verlag Gm

[ 16 ] J. Rybczynski , U. Ebels , M. Giersig , Colloids Surf. A 2003 , 219 , 1 . [ 17 ] M. J. Allen , V. C. Tung , L. Gomez , Z. Xu , L. Chen , K. S. Nelson , C.

Zhou , R. B. Kaner , Y. Yang , Adv. Mater. 2009 , 21 , 2098 . [ 18 ] T. R. Hendricks , J. Lu , L. T. Drzal , I. Lee , Adv. Mater. 2008 , 20 ,

2008 . [ 19 ] X. Liang , Z. Fu , S. Y. Chou , Nano. Lett. 2007 , 7 , 3840 . [ 20 ] D. Li , W. Windl , N. P. Padture , Adv. Mater. 2008 , 21 , 1243 . [ 21 ] X. Liang , A. S. P. Chang , Y. Zhang , B. D. Harteneck , H. Choo , D. L.

Olynick , S. Cabrini , Nano Lett. 2009 , 9 , 467 . [ 22 ] Z. Wei , D. E. Barlow , P. E. Sheehan , Nano Lett. 2008 , 8 , 3141 . [ 23 ] M. Hegner , P. Wagner , G. Semenza , Surf. Sci. 1993 , 291 , 39 . [ 24 ] C. Di , D. Wei , G. Yu , Y. Liu , Y. Guo , D. Zhu , Adv. Mater. 2008 , 20 ,

3289 . [ 25 ] L. Song , L. Ci , W. Gao , P. M. Ajayan , ACS Nano 2009 , 3 , 1353 . [ 26 ] Y. Zhou , Q. Bao , B. Varghese , L. Tang , T. Khim , C. Sow , K. Loh , Adv.

Mater. 2010 , 22 , 67 . [ 27 ] a) X. Li , X. Wang , L. Zhang , S. Lee , H. Dai , Science 2008 , 319 , 1229 ;

b) L. Jiao , L. Zhang , X. Wang , G. Diankov , H. Dai , Nature 2009 , 458 , 877 ; c) X. Wang , Y. Ouyang , X. Li , H. Wang , J. Guo , H. Dai , Phys. Rev. Lett. 2008 , 100 , 206803(4) ; d) Y. Ouyang , X. Wang , H. Dai , J. Guo , Appl. Phys. Lett. 2008 , 92 , 243124(3) .

[ 28 ] a) D. C. Bell , M. C. Lemme , L. A. Stern , J. R. Williams , C. M. Marcus , Nanotechnology 2009 , 20 , 455301(5) ; b) M. C. Lemme , D. C. Bell , J. R. Williams , L. A. Stern , B. W. H. Baugher , P. Jarillo-Herrero , C. M. Marcus , ACS Nano 2009 , 3 , 2674 .

[ 29 ] a) J. C. Meyer , C. O. Girit , M. F. Crommie , A. Zettl , Appl. Phys. Lett. 2008 , 92 , 123110(3) ; b) M. D. Fishchbein , M. Drndi , Appl. Phys. Lett. 2008 , 93 , 113107(3) .

[ 30 ] D. Pickard , L. Scipioni , http://www.zeiss.de/C1256E4600307C70/EmbedTitelIntern/AN_ORION_PLUSGraphene/$File/AN_ORION_PLUS_Graphene.pdf, 2009 (last accessed Mai 2010).

bH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 3615–3620

Documents

Making Patterns on Graphene