This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: 143.248.110.214 This content was downloaded on 12/12/2013 at 03:51 Please note that terms and conditions apply. Device-oriented graphene nanopatterning by mussel-inspired directed block copolymer self- assembly View the table of contents for this issue, or go to the journal homepage for more 2014 Nanotechnology 25 014008 (http://iopscience.iop.org/0957-4484/25/1/014008) Home Search Collections Journals About Contact us My IOPscience
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IP Address: 143.248.110.214
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Device-oriented graphene nanopatterning by mussel-inspired directed block copolymer self-
assembly
View the table of contents for this issue, or go to the journal homepage for more
2014 Nanotechnology 25 014008
(http://iopscience.iop.org/0957-4484/25/1/014008)
Home Search Collections Journals About Contact us My IOPscience
Seokhan Park, Je Moon Yun, Uday Narayan Maiti, Hyoung-Seok Moon,Hyeong Min Jin and Sang Ouk Kim
Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS),Materials Science and Engineering, KAIST, Daejeon, 305-701, Republic of Korea
Received 29 May 2013, in final form 4 July 2013Published 11 December 2013
AbstractDirected self-assembly of a block copolymer is successfully employed to fabricatedevice-oriented graphene nanostructures from CVD grown graphene. We implementedmussel-inspired polydopamine adhesive in conjunction with the graphoepitaxy principle totailor graphene nanoribbon arrays and a graphene nanomesh located between metal electrodes.Polydopamine adhesive was utilized for facile and damage-free surface treatment tocomplement the low surface energy of pristine graphene. Our process minimizes the damageto the ideal graphitic structures and electrical properties of graphene during the nanopatterningprocess. Multi-channel graphene nanoribbon arrays and a graphene nanomesh weresuccessfully fabricated between metal electrodes.
(Some figures may appear in colour only in the online journal)
1. Introduction
The unprecedented peculiar structures and properties ofgraphene, including a high carrier mobility and atomicscale dimensions, offer valuable opportunities to comple-ment the inherent limitations of silicon based electronics[1, 2]. Unfortunately, pristine graphene possesses semi-metallic characteristics without an electronic energy bandgap,which is generally disadvantageous for electronic appli-cations. How to attain semiconducting properties with anappropriate energy bandgap is a principal challenge in currentgraphene research [3, 4]. From the early days of grapheneresearch, sub-10-nm scale patterning of graphene has beensuggested as a straightforward route to the preparation ofsemiconducting graphene [5–10].
Directed self-assembly of block copolymers is anemerging bottom-up technology for sub-10-nm scale nanopat-terning [11–17]. Spontaneous microphase separation ofchemically immiscible polymer blocks generates denseperiodic arrays of self-assembled nanodomains in thin films,which can be exploited for lithographic templates [18, 19].
To date, several research groups have exploited blockcopolymer lithography for graphene nanopatterning [20–23].Self-assembled hexagonal nanodot and parallel nanowiretemplates have been employed for nanomesh and nanoribbonfabrication, respectively. Unfortunately, large-area nanopat-terning of low surface energy pristine graphene raisesformidable technological issues. Moreover, an effectivenanofabrication process for device integration is anothersignificant challenge for eventual device applications.
Herein we report a versatile route to nanopatternedgraphene structures, including graphene nanoribbon arraysand a graphene nanomesh, by means of mussel-inspireddirected block copolymer self-assembly. Previous works ongraphene nanopatterning generally require SiO2 or anothersacrificial layer formation on the graphene surface tocomplement the low surface energy and chemical inertness ofpristine graphene. Such sacrificial layer formation is usuallyaccompanied by a complicated multistep nanopatterningprocess that involves harsh chemical etching, which cansignificantly damage the ideal structures and properties ofpristine graphene. Recently, our research group developed
mussel-inspired block copolymer lithography based onpolydopamine adhesives as a universal nanopatterningprocess for low surface energy materials, including Teflon,pristine graphene and gold [24, 25]. In this work, wesuccessfully integrate mussel-inspired block copolymerlithography with a directed self-assembly principle forlarge-area nanopatterning of graphene for device-orientedstructures. We deposit polydopamine onto a graphene channelconfined between metal electrodes via a mild aqueous solutionprocess. Following a straightforward surface treatment witha polymer brush neutralized both a bottom graphene surfaceand metal electrode side walls without any damage to theunderlying graphene layer. Directed self-assembly of blockcopolymer thin films within this neutralized trench generateda vertical lamellar array aligned across the two metalelectrodes by means of graphoepitaxy [26–33]. Subsequentselective etching replicated a block copolymer self-assemblednanostructure onto the underlying graphene layer such thathighly aligned graphene nanoribbon arrays were created.Significantly, the resultant graphene nanostructures are highlycompatible with a current field-effect transistor (FET) devicestructure in which semiconducting channels directly connectmetal electrodes.
2. Experimental details
2.1. Fabrication of graphene field-effect transistor structures
A positive tone photoresist AZ5214 (Clariant, US) wasdeposited on graphene/SiO2/Si substrates (graphene square,KR, 10 mm2, 300 nm SiO2) by spin casting performed for30 s at 3000 rpm and soft baked at 100 ◦C for 1 min toevaporate the residual solvent. The photoresist layer wasexposed to an I-line source (Midas/MDA-6000 DUV, KR;wavelength: 365 nm; 9.5 mW cm−2) through a patternmask. Pattern development was performed by immersing thephotoresist film into AZ MIF300 (Clariant, US) for 30 s andthorough washing with water for 30 s. Cr/Au (10 nm/200 nm)layers were deposited by E-beam evaporation to form sourceand drain electrodes. The remaining photoresist layer wasremoved by washing in acetone after the metal electrodedeposition.
2.2. Mussel-inspired surface modification
The fabricated graphene FET structure was immersed in adilute aqueous solution of dopamine (1 mg ml−1), bufferedto a pH typical of marine environment (10 mM Tris-HCl,pH = 8.0) for 1 h [34].
For surface neutralization, a 250 nm thick hydroxylgroup terminated poly(styrene-ran-methyl methacrylate) P(S-r-MMA) brush layer was spin-cast onto the dopamine treatedFET structures and thermally annealed in a vacuum. Aftera sufficient thermal reaction time, excess polymer brushmolecules were thoroughly spin-washed [35].
2.3. Directed self-assembly of block copolymers
After surface modification, thin films of diblock copolymerwere spin-coated onto the graphene area exposed betweenthe metal electrodes. While a 200–220 nm thick symmetricpolystyrene-block-poly(methyl methacrylate) (PS-b-PMMA,Mn: 51 kg mol−1 and lamellar period: 32 nm) was used forthe graphene nanoribbon array, an asymmetric PS-b-PMMA(Mn: 67 kg mol−1 and cylinder spacing: 20 nm) wasemployed for the graphene nanomesh. Thermal annealing wasconducted at 250 ◦C for the directed self-assembly of blockcopolymers into highly ordered equilibrium nanostructures.After the thermal assembly, the PMMA nanodomains in blockcopolymer thin films and the underlying graphene layerswere selectively etched by O2 plasma treatment to patterntransfer the block copolymer self-assembled nanostructuresinto graphene layers.
2.4. Characterization
The nanoscale self-assembled morphologies of block copoly-mers and the created metal or photoresist patterned structureswere imaged using a Hitachi S-4800 scanning electronmicroscope (SEM) with a field emission source at 1 kV. Thetransfer characteristics were measured at room temperatureusing a probe station and an HP 4145B semiconductorparameter analyzer.
3. Results and discussion
3.1. Fabrication of a graphene field-effect transistor
The overall process for a multi-channel graphene nanoribbonarray is briefly described in figure 1. Source and drainelectrodes of Cr/Au (10 nm/200 nm) were deposited atthe graphene/SiO2/Si substrate by area-selective E-beamevaporation via a lithographically defined photoresist pattern.Figure 2(a) shows the photoresist pattern for metal electrodedeposition. Figure 2(b) shows an optical micrograph ofthe resultant graphene channel confined between two metalelectrodes. The channel length and width were 1 and25 µm, respectively. It is noteworthy that a stable bandgapopening of graphene nanoribbon for room temperaturerequires sub-10-nm scale patterning. Directed self-assemblyof block copolymers not only enables the ultrafine nanoscalepatterning in such a sub-lithographic scale, but also generatesa highly aligned dense periodic array connecting two metalelectrodes. The vertical side walls of metal electrodes playthe role of a structure guiding confinement for graphoepitaxy.Thus, the metal electrode height was optimized for thegraphoepitaxy effect.
3.2. Mussel-inspired directed self-assembly of blockcopolymers
In our approach, a chemically inert, low energy graphenesurface can be effectively modified by mussel-inspiredpolydopamine treatment without any damage to the graphene
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Nanotechnology 25 (2014) 014008 S Park et al
Figure 1. Schematic procedure for graphene nanopatterning by mussel-inspired directed block copolymer self-assembly.
Figure 2. Multi-channel graphene nanoribbon array aligned across metal electrodes. (a) SEM image of a photoresist pattern for metalelectrode deposition. (b) SEM image of a continuous graphene channel confined between two metal electrodes. (c) SEM image of aPS-b-PMMA vertical lamellar array highly aligned across the two metal electrodes by means of graphoepitaxy. (d) SEM image of amulti-channel graphene nanoribbon array formed across metal electrodes.
surface. Simple immersion in a buffered aqueous dopaminesolution (1 mg ml−1, 10 mM Tris-HCL, pH = 8.0) depositeda nanometer-scale thickness polydopamine layer onto boththe bottom graphene channel and the gold electrode surfaces.After polydopamine treatment, a hydroxyl group terminated
P(S-r-MMA) brush layer was spin-cast onto the entire surfacearea. Subsequent thermal annealing induced a covalentreaction between the hydroxyl terminal groups of the polymerbrush and catechol groups at the polydopamine layer. Theresultant brush-treated trench surface was chemically neutral
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Nanotechnology 25 (2014) 014008 S Park et al
Figure 3. Transfer characteristics of field-effect transistors employing a graphene channel between metal electrodes. (a) Nonpatternedgraphene channel characteristics. (b) Multi-channel graphene nanoribbon array characteristics.
(balanced identical surface tension) to PS and PMMAcomponents with the optimized chemical composition of therandom copolymer. Any excessive polymer brush moleculeswere thoroughly spin-washed by toluene. Finally, symmetricPS-b-PMMA (Mn: 51 kg mol−1 and lamellar period: 32 nm)thin films with thicknesses of 200–220 nm were spin-castin the neutralized trenches. Subsequent thermal annealingtriggered the directed self-assembly of block copolymer thinfilms following the graphoepitaxy principle.
3.3. Fabrication of graphene nanoribbon arrays
Figure 2(c) presents an SEM image of the resultantPS-b-PMMA vertical lamellar array highly aligned acrosstwo metal electrodes [28, 29]. As a result of surfaceneutralization of the bottom graphene and metal electrodeside walls, the lamellar array becomes oriented perpendicularto the bottom and sidewalls simultaneously, generating anidealized nanoscale multi-channel structure. UV radiationand subsequent O2 reactive ion etching (RIE) selectivelyremoved the PMMA lamellar domains in the block copolymerthin films. The remaining PS nanotemplate was usedas an etching mask for the underlying graphene layer.O2 RIE at a 100 W RF power completely etched theunderlying graphene layer to replicate the self-assembledblock copolymer nanochannel structures. Figure 2(d) showsthe resultant graphene nanoribbon array formed across themetal electrodes.
3.4. Electrical characterization
Unlike unipolar silicon CMOS devices, graphene FETis known to exhibit ambipolar conduction, which offersswitchability from p-type to n-type with the gate biasmodulation [1]. The typical ambipolar conduction wasobserved for FETs with nonpatterned graphene channels(figure 3(a)). The drain current (IDS) first decreased andthen increased with the gate voltage (VG). The voltagefor minimum current is defined as the Dirac point.Due to the electronegative moisture or oxygen molecules
inevitably absorbed on bare graphene surfaces in ambientconditions, the Dirac position was located in the positive VGrange, demonstrating apparent p-doped behavior. Figure 3(b)shows the transfer characteristics of multi-channel graphenenanoribbon FETs prepared by replicating a lamellar blockcopolymer self-assembled morphology. Due to the highlyelectronegative catechol groups in the polydopamine layer andthe oxygen functionalities generated at the O2 RIE etchednanoribbon edge, the Dirac point is out of the measurementrange, indicating a highly p-doped characteristic. Furtherprocessing optimization involved with the effective removalof the organic polydopamine residue and edge chemistrycontrol at the etched graphene is required for detailedelectrical characterization and device applications of thesemiconducting graphene nanoribbon arrays.
3.5. Fabrication of a graphene nanomesh
Graphene nanomesh has been introduced as anotherpromising graphene based nanostructure [20, 22, 23].In our approach, a graphene nanomesh structure couldalso be readily prepared by employing self-assembledhexagonal cylinder nanostructures of block copolymers asnanotemplates, instead of lamellar nanostructures. Unlikediscontinuous and discrete graphene nanoribbon structures,graphene nanomesh may offer a continuous semiconductingthin film, which can be utilized for transparent andflexible electronic device fabrication via a conventionalsemiconductor process. The fabrication process for graphenenanomesh is consistent with that of graphene nanoribbonarrays except for the employed block copolymer self-assembled structure. We employed asymmetric PS-b-PMMA(Mn: 67 kg mol−1 and cylinder spacing: 20 nm), whichforms hexagonal cylinder self-assembled nanostructures.Mussel-inspired directed self-assembly of the asymmetricblock copolymer generated hexagonal arrays of verticalnanocylinders on a graphene channel. Further O2 RIEreplicated the vertical nanocylinder morphology onto theunderlying graphene layer and successfully created agraphene nanomesh with a hexagonal antidot structure.
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Nanotechnology 25 (2014) 014008 S Park et al
Figure 4. Graphene nanomesh fabrication between metal electrodes. Optical microscope and SEM images of a hexagonal nanoporousblock copolymer template formed on a graphene channel between (a) 1 µm and (b) 3 µm gap distance electrodes.
Figure 4 shows optical microscope and SEM images of theresultant graphene nanomesh fabricated between two metalelectrodes. The gap distances between the metal electrodes are1 and 3 µm for figures 4(a) and (b), respectively.
4. Conclusion
We have demonstrated that directed self-assembly ofblock copolymers in conjunction with mussel-inspiredpolydopamine universal adhesive accomplished a highlyeffective fabrication process for graphene nanostructures,including graphene nanoribbon arrays and a graphenenanomesh. Significantly, our mussel-inspired nanopatterningdoes not require complicated process steps involving aninorganic sacrificial layer formation and minimizes the harshchemical etching process that can severely damage the idealgraphitic structures and outstanding material properties ofpristine graphene. Further advances of this approach forsub-10-nm scale feature sizes and an appropriate choiceof graphene edge control should allow the production ofsemiconducting graphene nanostructures, which are in greatdemand for diverse device applications, including flexibleand transparent electronic/optoelectronics, highly sensitivenanoscale sensors and energy storage devices.
Acknowledgment
The authors would like to acknowledge the Institute for BasicScience (IBS) in Korea for financial support.
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