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Journal ofMaterials Chemistry C

FEATURE ARTICLE

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Continuous and h

DfaUriEnrPEMi

and scalable nanomanufacturingrollable nanofabrication and temthe hybrid functional nanostructtubes, graphene, and metal oxidtronic, and energy conversion devi

aDepartment of Mechanical Engineering, Uni

USA. E-mail: [email protected]; Fax: +1 734bDepartment of Electrical Engineering and C

Ann Arbor, MI 48109, USAcSchool of Mechanical Engineering, Kyungp

Republic of Korea

† Current address: Molecular Imprints, In

Cite this: J. Mater. Chem. C, 2013, 1,7681

Received 15th May 2013Accepted 18th July 2013

DOI: 10.1039/c3tc30908h

www.rsc.org/MaterialsC

This journal is ª The Royal Society of

igh-throughput nanopatterningmethodologies based on mechanical deformation

Jong G. Ok,ab Se Hyun Ahn,†a Moon Kyu Kwakbc and L. Jay Guo*ab

This feature article provides an overview of several mechanical-based micro- and nanopatterning

technologies that can achieve continuous and high-throughput fabrication of various sub-wavelength

structures without resorting to the conventional optical lithography technique. These include a

template-based and versatile roll-to-roll nanoimprint lithography technique, cost-effective dynamic

mould sweeping patterning as well as mould-free patterning methods. Examples of demonstrated and

potential applications in optoelectronics and photonics are also discussed.

Introduction

There has been an increasing demand for sub-wavelengthstructures in optoelectronics and photonics.1–3 Fabrication ofsuchmicro/nano-scale structures has traditionally been done byoptical lithography due to its reliability andmaturedprotocols.4,5

However, the diffraction limit in photolithography ultimatelyrestricts the attainable feature sizes. Also, its throughput and

r Jong G. Ok is a researchellow in Electrical Engineeringnd Computer Science at theniversity of Michigan. Heeceived B.S. and M.S. degreesn Mechanical and Aerospacengineering from Seoul Natio-al University in 2002 and 2007,espectively, and received hish.D. degree in Mechanicalngineering at the University ofichigan in 2013. His research

nterests involve the continuoustechnologies highlighted by

plate-free nanopatterning andures comprising carbon nano-e nanowires, for optical, elec-ces.

versity of Michigan, Ann Arbor, MI 48109,

763 9324; Tel: +1 734 647 7718

omputer Science, University of Michigan,

ook National University, Daegu 702-701,

c., Austin, TX 78758, USA.

Chemistry 2013

limitation in scalable processing cannot address the emergingneeds of large-area and low-cost manufacturing of micro/nano-engineered devices, especially in a continuous process. In thisregard, a series of efforts have been made to search for newformats to extend the photolithography-based technique tocontinuous patterning using a roller-assisted process;6–8 to breakthe diffraction limit by using laser interference9 or so lithog-raphy,10 or alternative nano-scale feature fabrication methodol-ogies such as the micro/nano-scale patterning using a blockcopolymer array11,12 or controlled wetting,13 pattern-inducedpolymer self-organization14 plasmonic nano-ruling,15 andNanoImprint Lithography (NIL).16,17 Not only opening a newroute for the fabrication of small-scale patterns beyond the limitof conventional photolithography, these methods also haveexplored extraordinary properties ranging from supramolecularphenomena18 to macroscale functionalities like colour

Dr Se Hyun Ahn is currently asenior research engineer atMolecular Imprints. He receiveda B.S. degree in MechanicalEngineering from YonseiUniversity in 2001, M.S. degreein Mechanical and AerospaceEngineering from Seoul Natio-nal University in 2003, andPh.D. degree in MechanicalEngineering from the Universityof Michigan in 2010. Hisresearch interests include high-

throughput nanopatterning technology based on nanoimprintlithography and nano-optoelectronics for display applications.

J. Mater. Chem. C, 2013, 1, 7681–7691 | 7681

http://dx.doi.org/10.1039/c3tc30908h

http://pubs.rsc.org/en/journals/journal/TC

http://pubs.rsc.org/en/journals/journal/TC?issueid=TC001046

Fig. 1 Schematic illustration of nanopatterning methods based on mechanicaldeformation: (a) conventional NIL; (b) Roll-to-Roll/Roll-to-Plate NIL (R2R/R2P NIL)makes use of the rolling of a flexible master mould to continuously imprint thepattern; (c) Dynamic Nano-Inscribing (DNI) and NanoChannel-guided Lithography(NCL) use a cleaved edge to seamlessly create the grating pattern on a movingsubstrate; (d) Localized Dynamic Wrinkling (LDW) employs a sliding flat edge toinduce spontaneous buckling of a thin stiff layer to break into the grating pattern;(e) Vibrational Indentation-driven Patterning (VIP) relies on vertical vibration of aflat tool edge to periodically indent a line pattern on a moving substrate.

Journal of Materials Chemistry C Feature Article

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ltering.19 However only certain techniques can be conguredfor large-area and continuous fabrication.

In particular, NanoImprint Lithography (NIL) provides aunique solution to achieve sub-10 nm spatial resolution20 withhigh precision and great reproducibility. In NIL (Fig. 1a), a rigid‘mould’ containing the surface relief patterns imprints thedesired pattern onto a target substrate under controlled pressure,aided by heat and/or UV illumination. NIL has provided anextensive nanopatterning solution for many applications bymechanically dening the sub-micron period patterns beyondthe optical diffraction limit. For example, NIL has uniquelyenabled the fabrication of nanoscale metal gratings that canfunction as wire-grid polarizers,21,22 transparent electrodes,23,24

and plasmonic enhanced charge-collecting layers in photovol-taics,25–27 as well as the nanogratings in themetal-insulator-metal(MIM) stack that has led to the novel colour lters for displayapplications.19,28,29 NIL has also been utilized to pattern func-tional nanostructures such as graphene30 and biomaterials.31

Traditional NIL is capable of wafer-scale processing atnanoscale resolution with relatively low cost,17 yet the currentprocess and throughput in traditional NIL is still far frommeeting the demands of many practical applications, especiallyin photonics, biotechnology, and organoelectronics.32 Namely,in typical NIL, the replica area is limited by the master mouldsize because the patterning process is essentially ‘stamping’.

In this article, we review several continuous and high-throughput patterning techniques based on direct mechanicaldeformation, which are capable of more scalable and fasterfabrication of sub-wavelength patterns than conventionaltechniques; some can be done with only a small area of mastermoulds, or even without any pre-dened master patterns. Wewill also discuss examples that illustrate how thesemethods canbe applied to the high-throughput fabrication of various elec-tronic, energy conversion, optical, and photonic devices.

Specically, we rst present the Roll-to-Roll (R2R) NIL (R2RNIL)33,34 that has been developed utilizing the rolling of a exiblemaster mould to continuously imprint the replica pattern on

Dr Moon Kyu Kwak is an assis-tant professor in the School ofMechanical Engineering atKyungpook National University.He received Ph.D. degree in Me-chanical and Aerospace Engi-neering at Seoul NationalUniversity in 2011. He moved tothe University of Michigan forpostdoctoral research with Prof.L. Jay Guo, working on the roll-based continuous fabricationsystems. His research interests

include the development of continuous novel patterning systemsand applications on display devices and the investigation of bio-inspired smart surfaces such as water collecting surfaces and dryadhesives.

7682 | J. Mater. Chem. C, 2013, 1, 7681–7691

either a exible (R2R) or rigid (Roll-to-Plate; R2P) substrate(Fig. 1b). R2R/R2P NIL provides a much improved throughputandexpanded replica areawhile preservingnanoscale resolution,

Dr L. Jay Guo is a professorof Electrical Engineering andComputer Science at the Uni-versity of Michigan, with jointappointments in MechanicalEngineering, MacromolecularScience and Engineering, andApplied Physics. His group'sresearch includes polymer-basedphotonic devices and sensorapplications, organic photovol-taics, plasmonic nanophotonics/metamaterials, nanoimprint-

based and roll-to-roll nanomanufacturing technologies. Hereceived his Ph.D. in Electrical Engineering from the University ofMinnesota in 1997.

This journal is ª The Royal Society of Chemistry 2013


Feature Article Journal of Materials Chemistry C

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which can be capitalized upon for various applications. Forinstance, the nanograting structures patterned by R2R/R2P NILcan be adopted to make large-area metal wire-grid polarizers(MWGPs)33,35 and organic photovoltaic cells (OPVs).36 Thisprocess can essentially produce any desired patterns, and we willshow R2R-imprinted sub-wavelength scale dot patterns on theMIM stack to achieve large-area exible plasmonic structures.37

Wemove on to introduce Dynamic NanoInscribing (DNI)38 andNanoChannel-guided Lithography (NCL)39 both of which adoptthe horizontal sliding of a cleaved edge of a small area of a rigidmaster mould to inscribe seamless and indenitely long nano-grating patterns (Fig. 1c). A variety of metal or polymer materialscan be patterned with the feature size down to sub-50 nm athigh speed (�10 cm s�1). DNI is conducted either at ambienttemperature or with a brief heating time of tens of microsec-onds, thereby minimizing damage to UV- or thermo-sensitivefunctional materials.

NCL further realizes continuous patterning of nanogratingswith a higher aspect ratio than those made by DNI. It works byemploying the UV-curable liquid resist free from elastic recoveryon the mould sweeping process, which then smoothly extrudesfrom the nanochannels dened on a slice of a master gratingmould. A successful NCL makes use of the selective wetting andself-stabilizationof thedelineated liquid linesuntil crosslinking.

Moving forward, Localized Dynamic Wrinkling (LDW)40 hasopened a ‘template-free’ fabrication of grating patterns(Fig. 1d). In LDW, a ‘at’ tool edge slides over a metal-coatedpolymer substrate under conformal contact, which inducesspontaneous wrinkling and buckling on the thin metal layer tobreak it into a line pattern. Removing the need for pre-fabri-cation of master moulds may signicantly improve the overallprocess throughput and reduce the total cost.

As another method free from master pattern preparation,Vibrational Indentation-driven Patterning (VIP)41 creates sub-wavelength grating structures via the periodic indentations

Fig. 2 (a) Schematic descriptions of R2R NIL mode (top) for flexible substrates andresist is fed into the R2R NIL system where a roll bearing a flexible mould continuoupatterns fabricated by R2R/R2P NIL on (b) a PET film and (c) a glass plate; (d) 60-coAmerican Chemical Society).


driven by the vertical vibration of a at tool edge onto any soermaterials (Fig. 1e). VIP enables the formation of period-tunablegratings by real-time control of the vibration frequency and/orthe substrate feeding speed. It can also tune the blazed gratingangle by adjusting the tool tilting angle, which can be practicallycongured to the MWGPs aer shadow-evaporation of metal aswill be demonstrated later.

Patterning by continuous rolling: R2R/R2PNIL

The conventional NIL has achieved high resolution nano-patterning and found a range of potential applications. Thethroughput and area of patterning of the NIL technique can bescaled up by adopting a exible and rollable master mouldwhich can continuously imprint over an indenitely longsubstrate coated with resists. This continuous roll-based NILprocess,33,42 as schematically described in Fig. 2a, can becongured to R2Rmode for exible lms (e.g. polymers; Fig. 2b)or R2P mode for rigid substrates (e.g. glass plates; Fig. 2c) (R2R/R2P NIL will be denoted simply as R2R hereaer).

InR2R,aexiblemouldcontainingdesiredpatterns ispreparedand wrapped around a roll. Either a exible or rigid substratecoated with a resist mounted on another roll or a at conveyer isthen fed into the contact zone where a mould continuouslyimprints the replica pattern onto the substrate under conformalpressure. The instant UV curing is followed at the outlet of themould–substrate contact zone to nish the patterning.

A successful and reliable R2R can be achieved by choosing thesuitable mould and resist materials and appropriate, continuousanduniformresist coatingmethods. First, themoulds forR2Rnotonly should beexible enough towrap around a roll but also needto have sufficient modulus to durably imprint other materials.Also of great signicance is a low surface energy for a clean andcontinuous demoulding process. Meeting these requirements,

R2P NIL mode (bottom) for rigid substrates. A substrate coated with a UV-curablesly imprints the pattern which is cured by UV light after mould release; large-areampatible R2R/R2P NIL system (reproduced from ref. 34 with permission from the

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uoropolymers17,33 such as ethylene tetrauoroethylene (ETFE)and peruoroalkoxy (PFA), thermally curable polydimethylsilox-ane (PDMS),37 and UV-curable and low surface-energy materialssuchasuoro-SSQ43 are suitable candidates asmouldlms. In thecase of thermoplastics such as ETFE, the exible mould can befabricated by thermal embossing at a temperature above thematerial's glass transition temperature (Tg). A exible Ni mouldcan also be used by the electroforming process against a pre-fabricated master,44 and used for thermal R2R NIL due to itsendurance and stability.

As for resist materials, typical thermoplastics can all be used ifimprinting is done at a temperature well above the Tg of thematerial. However, the high viscosity of the material even atelevated temperature may still require signicant pressure forsuccessful imprinting.17Ontheotherhand, liquid-stateUV-curablepolymers such as epoxysilicone45 and epoxy-silsesquioxane (SSQ)43

are overall great choices for rapid and continuous patterning atambient temperature. A certain level of viscosity at uncured statusis desired to ensure that the as-coated resist can maintain auniform thin lm.46 There are several additional techniques tooffer further aid for the patterned resist to remain on the substratewithout being detached from themould; themould surface can bemodied tohave lower surface energy, the substrate surface can betreated with an adhesion promoter prior to the resist coating,42 orthe anti-sticking agent can be added to resist material syntheses.22

Another important aspect is the continuous and uniformresist coating on a large-area substrate. In this regard, roll-dipping or continuous drop-casting followed by a doctor-bladescraping,47,48 or on-demand droplet dispensing49 are attractivealternatives for continuous and uniform coating, which can bereadily integrated into the R2R system as suggested in the rightside of the R2R scheme in Fig. 2a.

Fig. 3 (a) R2R fabrication of a flexible wire-grid polarizer film where the resistcoating, pattern imprinting, and metal evaporation processes can all be contin-uously performed along the integrated R2R system (reproduced from ref. 33 withpermission from John Wiley and Sons); (b) R2R fabrication of a large-area plas-monic IR filter by continuously patterning the sub-wavelength dot array on anMIM-stacked flexible substrate (reproduced from ref. 37 with permission from theAmerican Institute of Physics).

Applications

With its simple and continuous patterning principle, R2R canbe widely used for the scalable and high-throughput fabricationof various electronic and photonic devices. By depositing a thinmetal layer (e.g. Al or Ag) on the R2R/R2P-fabricated gratingstructures, MWGPs33,35 can be fabricated. The metal-depositingmodule can be essentially integrated into the R2R systemthrough continued feeding of the R2R-patterned substrate intothe evaporation chamber as shown in Fig. 3a. Here the metaldeposition can be performed either at normal33,35 or tiltedangles22,50 depending on the design and performance.

A exible and large-area plasmonic metamaterial lm forbroadband IR lters has also been recently demonstrated byR2R patterning of sub-wavelength patterns on a MIM-stackedpolymer lm.37 The plasmonic MIM structure comprising Al–SiO2–Al is successfully fabricated on a exible polyethyleneterephthalate (PET) substrate where the top Al layer is contin-uously patterned into the sub-wavelength scale metal disk arrayvia R2R (Fig. 3b). The patterned metal disks having varyingdiameters and sub-micron spacing lead to the desired broad-band IR ltering performance at the designed dual-band.

It is worth noting that in general R2R is highly desirable forhigh-throughput manufacturing of large-area structures and/or

7684 | J. Mater. Chem. C, 2013, 1, 7681–7691

on exible platforms. There are many other applications thatcan be directly fabricated and/or integrated with R2R, includingprintable/exible electronics,51 scalable fabrication of large-areasolar cells,52 and functional organic materials.53 Carbon nano-tubes (CNTs)54 and graphene55,56 of emerging interest assustainable and multipurpose nanomaterials57 can also becontinuously processed via R2R for transparent electrodes andelectronic templates. Other reported applications includetransistors,58,59 sensors,60 light sources,61 data storage,62 fuelcells,63 and rectennas;64 and much more will likely follow.

Challenges

One of the challenges underlying R2R is the fabrication of alarge-area original exible mould for wrapping the roller.




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Usually starting from the small-area fabrication of masterpatterns on a rigid substrate (e.g. Si) by means of laserinterferometry or electron-beam (e-beam) writing, the mouldmust be scaled up to the desired area through stepping ortiling processes, followed by the transfer onto a exible lm.In such an approach, there is an inevitable seam regionwhere the two ends of the exible mould meet on the roller,which interrupts the otherwise continuous patterns. Effortsare being made to minimize the seam; for example, aninnovative approach of continuous e-beam lithography on arotating roller has been recently developed65 for the fabrica-tion of exible wire-grid polarizers by R2R. In what follows,we present alternative methodologies to address these issuesto realize seamless patterning without the need for large-areamoulds.

Fig. 4 DNI on (a) a rigid substrate and (b) a flexible or curved substrate, where acleaved mould edge inscribes the seamless grating pattern on a moving polymersubstrate under 2D contact; (c) 700 nm-period grating pattern fabricated by DNIon a long polycarbonate strip. The inset shows the scanning electron microscopy(SEM) image of a cross-section of the fabricated grating structure (reproducedfrom ref. 38 with permission from the American Chemical Society).

Patterning by horizontal mould sweep: DNI/NCL

In this section, we introduce Dynamic Nano-Inscribing (DNI)38

and NanoChannel-guided Lithography (NCL)39 techniques fordynamically creating seamless, truly continuous sub-wave-length scale linear patterns with feature sizes down to sub-50nm at high speed. The basic principle of DNI and NCL is toperform the horizontal ‘sweeping’ of a sliced edge of a mouldcontaining desired grating patterns onto the substrate at a tiltedangle, thereby mechanically inscribing the surface reliefpatterns on a solid substrate (DNI; Fig. 4) or extruding the liquidstructures to be set by crosslinking (NCL; Fig. 5).

DNI38 relies on the plastic deformation of materials to formpatterns in a polymer lm.17 Different from conventional NILthat relies on the viscous ow of polymer material and requiringa large-area stamp, the deformation of a substrate in DNI takesplace over a very small contacting region where the sharp edgeof a tilted Si mould engages the polymer surface; this two-dimensional (2D) processability enables dynamic patterningnot only on a normal at substrate (Fig. 4a) but on a exible orcurved surface (Fig. 4b). Continuous and seamless linearpatterns with innite length (Fig. 4c) on various polymers,metals or even hard materials such as ITO can be successfullymade by using very low applied forces (several Newtons), andwith a speed of �10 cm s�1 drastically faster than other nano-patterning techniques. The localized heating of the mould–substrate contact region can help increase the aspect ratio of theDNI-ed structure.38

Despite the extensive applicability of DNI, the elasticrecovery66 of the plastically deformed solid surfaces aer therelease of mechanical force may limit the aspect ratio of thenal grating structure (see the inset of Fig. 4c for example),especially for small period gratings. This cannot satisfy the needfor some applications such as MWGPs where higher aspect ratioand well-dened cross-sectional proles are highly desir-able.22,67 Also, the mould wear caused by force concentration onthe edge when used many times oen requires re-cleaving of asharp edge.

To address these issues, NCL has been developed by adopt-ing a liquid layer as resist material to pattern by the dynamic


mould-sweeping (Fig. 5a), aiming to achieve high-speed andlow-cost fabrication of continuous nanograting structures.39

Here UV-curable SSQ can be used as a suitable liquid resistwhich is originally a viscous liquid polymer but can rapidlysolidify upon UV curing without signicant volume shrinkage.43

Whereas a mechanical force is required to ‘inscribe’ nano-patterns on a solid substrate by plastically deforming thematerial, a liquid resist can readily ‘inltrate’ the openings inthe mould grating under conformal contact supported by slightpressure. These ‘nanochannel-guided’ liquid streaks aresmoothly extruded from the contact region as the mouldtranslates along the surface, enabling continuous formation ofnanograting patterns free from elastic recovery. The contactpoint can be maintained at ambient or elevated temperature toadjust the viscosity of the liquid resist for optimal lling of thenanochannel features on the mould within the processingtime.68 UV exposure promptly cures the as-patterned liquidextruded from the nanochannels into the solidied nano-gratings with a well-retained prole. NCL can produce a seam-less nanograting having a higher aspect ratio (Fig. 5b) than theone inscribed on a solid substrate that inevitably undergoeselastic recovery (Fig. 5c).

To conduct successful NCL patterning, the substrate mate-rial should have a non-wetting property (e.g. PFA and ETFE) withrespect to the liquid resist used (Fig. 5d). The non-wettingbehaviour of the as-formed liquid lines against the topograph-ical grooves deformed on the underlying substrate (Fig. 5e)prevents the immediate reow and helps sustain a high

J. Mater. Chem. C, 2013, 1, 7681–7691 | 7685


Fig. 5 (a) Schematics of the NCL process where a UV-curable liquid-coatedsubstrate is engagedwith a cleavedmould edge and continuously delineated intothe grating pattern as the mould proceeds. Mould heating is applied to adjust theviscosity of liquid, thereby controlling the final grating geometry; comparison of200 nm period nanograting structures fabricated by (b) NCL on an SSQ-coatedPFA substrate and (c) DNI on a PFA substrate, showing that NCL can achievehigher aspect-ratio patterns due to the crosslinking of the liquid resist, which isfree from elastic recovery of the thermoplastic material used in DNI; (d) non-wetting behavior of SSQ on a flat PFA surface with a large contact angle; (e)formation of an isolated liquid SSQ nanostructure that can sustain its shape untilcuring against the grooved topology of a PFA substrate underneath; (f) SEMimage of the resulting nanograting structure with an inset showing the cross-section (reproduced from ref. 39 with permission from John Wiley and Sons).

Fig. 6 Applications of DNI/NCL to various feature fabrications: (a) concentriccircular nanogratings of the 700 nm period created by rotating the mould edgeon an ETFE substrate; (b) 700 nm period gratings inscribed on a curved surface ofcrosslinked SU8 blocks; (c) 200 nm period Ag nanograting fabricated by directlyinscribing on a Ag-coated polycarbonate film; (d) 200 nm period Au nanogratingswith sharp turns (reproduced from ref. 38 with permission from the AmericanChemical Society).


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aspect-ratio grating prole until fully cured by UV light(Fig. 5f).39 The nal geometry of liquid gratings can be tailoredby the processing temperature as well as the surface charac-teristics of the underlying solid substrate. Such a ‘direct-write’NCL patterning of liquid resists is a gentler process which canprolong the mould lifetime, and result in a much more faithfulpattern replication especially for small-period and high aspectratio structures.

Features and applications

As DNI/NCL can be conducted dynamically under the 2Dcontact, various features that are hardly fabricable throughconventional methods can be fabricated by DNI/NCL; thegrating patterns can be formed in nonlinear (e.g. circular)shapes (Fig. 6a) or conformally on curved surfaces (Fig. 6b).38

Also, the thin metal lm can be directly formulated to themetal grating structure by DNI, either in a straight (Fig. 6c) ora sharply turned fashion (Fig. 6d), which demonstratesthe optical metamaterial characteristics.38 Likewise, variousfunctional polymer layers can be seamlessly patterned byDNI/NCL.

Since in NCL an uncured resist remains liquid, the gratingmould ‘teeth’ can easily penetrate into a thin liquid resist layer,touching the underlying substrate surface at contact. As NCL

7686 | J. Mater. Chem. C, 2013, 1, 7681–7691

proceeds, this liquid layer can therefore readily break into thediscrete lines guided by the linearly transferring mould open-ings while the liquid in the area where the mould ‘hills’ keepconformal contact to the substrate underneath is scraped off.Hence, NCL is essentially a residual layer-free process, elimi-nating the post-etching process for the residual layer removalthat is required in many cases. Beneting from the dynamic 2Dprinciple as well as the residual layer-free feature, NCL can beutilized for, for instance, one-step fabrication of polymer ribwaveguides of smooth sidewalls where the NCL-ed liquid linefunctions as a waveguide core against the cleanly borderedsurrounding serving as undercladding.69 It is expected that theNCL-fabricated waveguide can signicantly lower the propaga-tion loss due to its smooth sidewall. Comparatively, the wave-guide fabricated by conventional NIL may suffer from a higherloss mostly originating from its sidewall roughness70 whichcannot be avoided in traditional microfabrication (e.g. RIE) and/or NIL stamping.

Challenges

Compared to conventional NIL and R2R, DNI/NCL requires avery small area (i.e. cleaved edge) of master moulds containingdesired micro/nano-patterns. Considering the typical featureheight is only a few hundreds of nanometers, the atness of thecleaved edge is very important to ensure a complete conformalcontact of all grating lines with the polymer substrates. Inaddition, fabricating a suitable master mould of large width isneeded for practical applications. For many applications thatrequire micro- or nanoscale patterns, but not necessarily peri-odic or regular arrays (e.g. superhydrophobic surfaces), devel-oping patterning methodologies without needing prefabricatedmoulds will be highly desirable and we will introduce one suchtechnique.




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Spontaneous buckling-driven patterning:LDW

Exploited in this technique is the spontaneous wrinkling andbuckling of a thin solid lm on a compliant substrate, whichcan be used to fabricate quasi-periodic grating patterns withouthaving to resort to any prepatterned masters. This novel tech-nique termed Localized Dynamic Wrinkling (LDW)40 can enablecontinuous formation of micro/nano-scale gratings out of a thinlm (e.g. metal) coated on common polymer substrates. Asschematically illustrated in Fig. 7a, the moving sharp edge of acleaved ‘at’ Si wafer locally creates a wrinkle in a metal-coatedpolymer surface followed by several others with rapidly

Fig. 7 (a) Schematics of LDW where a flat tool edge sliding over a thin film-coated substrate induces localized wrinkling of the thin stiff film on top of acompliant substrate; finite element modeling results comparing (b) the localizedwrinkle formation by the flat rigid edge sliding on a thin film surface and (c) thedistributed bulk wrinkling of a uniformly compressed bulk polymer; (d) sequentialsteps to create discrete metal gratings by LDW; (e) SEM image of an�1 mmperiodAu grating pattern created by performing LDW on the 50 nm thick Au thin filmdeposited on a PET substrate. The top and bottom insets show an enlarged SEMimage of (e), and an AFM profiling image of the 780 nm period pattern fabricatedby LDW on a 30 nm thick Au-coated PET film, respectively (reproduced from ref.40 with permission from the American Chemical Society).


decaying amplitude; the continuous motion of the sharp edgeproduces periodic grating patterns in a sequential manner.

LDW shares the basic principle of the buckling phenomenonof a thin stiff lm compressed on a compliant substrate underuniaxial stress,71 but it bears the challenge of producing suchstress over a large area (e.g. a substrate of innite length) byusing the moving edge of a cleaved Si wafer to exert stress on themetal lm coated on the polymer substrate along the uniquelydened movement direction. This stress sequentially generateslocalized wrinkles in the thin metal lm in a dynamic andcontrollable fashion. A nite element simulation shown inFig. 7b conrms that the wrinkle is localized just ahead of thesharp edge of the hard material pressing on the lm. As acomparison, Fig. 7c shows the simulated result of wrinkleformation distributed across the whole surface when the samelm is subject to a uniform stress. The difference between thelocalized wrinkling and the bulk wrinkling can be clearly seen.

Furthermore, the nature of the sequential formation oflocalized wrinkles along the at rigid (e.g. cleaved Si) edgemoving direction makes LDW easily applicable to a high-speedand continuous R2R patterning. In fact, it was observed that thepattern period is determined by the thickness of the stiff lmand the moduli of the lm and the substrate materials, butindependent of the speed of the moving edge in the experiment,as the latter is far below the elastic wave propagation speed in asolid substrate, which is believed to set the ultimate speed limitof the LDW process.40


LDW can directly generate nanoscale metal gratings on plasticsubstrates without using any prepatterned moulds. Fig. 7ddepicts how the LDW process creates discrete metal gratings.When a at rigid edge slides on the metal-coated substrate withsuitable pressure, localized wrinkles start to develop under theshear (friction) and normal forces and separate the thin metallayer into the periodic line structures. Fig. 7e shows the �1 mmperiod Au grating structure LDW transformed as such from the50 nm thick Au lm on PET; the enlarged view in the insetconrms the clearly cut Au lines. Interestingly, the period andgeometry of the patterned gratings can be determined bymaterial parameters such as the metal layer thickness and theYoung's moduli of metal and backing polymer.40 For example,performing LDW on thin Au lms of various thicknesses (i.e.10–100 nm) deposited on the plastic substrates such as PET andpolycarbonate led to the Au gratings of different periodsranging from the micron-scale to 120 nm,40 depending on theAu thickness and polymer material. The metallic structures canalso be transferred to other substrates to suit for differentapplications.

It is known that sub-wavelength high aspect-ratio metallicgratings can function as wire-grid polarizers.72 This applicationhas put quite stringent requirements on the dimensions andaspect ratio of the metallic wire grid.22,73 On the other hand,there are other applications which do not require a strict peri-odic arrangement of the metallic lines, for example, for trans-parent conductors used in many display and optoelectronic

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applications such as organic light-emitting diodes (OLEDs)27

and OPVs.23,25 The metal lines generated by the LDW processgenerally have small spacings, and hence, may not providesufficient transparency for transparent electrodes. However,this drawback can be overcome by uniaxially stretching theplastic substrate with LDW-formed metal lines. As illustrated inFig. 8a, the LDW-patterned metal grating that originally hadsmall openings is stretched orthogonally to the grating direc-tion with heat (raising the temperature higher than Tg of thepolymer substrate) to obtain larger openings. Fig. 8b shows theinitial grating pattern fabricated by LDW on a 50 nm Au-coatedPET substrate with openings less than 20 nm. 20% stretchingat 200 �C results in wider openings (Fig. 8c) and 30% stretchingproduces even larger openings (Fig. 8d) for increasedtransparency.

Challenges

Due to the nonlinearity and instability underlying the bucklingphenomena, it is still challenging to control the pattern geom-etry precisely (e.g. making perfectly straight lines). Besides,LDW only works for the bilayer systems comprising a stiff lmdeposited on a compliant substrate (e.g. metal/polymer), which

Fig. 8 Stretchable LDW to control the transmittance of the metal gratingpattern: (a) schematics of uniaxial stretching of an LDW-fabricated metal patternat the temperature beyond Tg to expand openings among metal lines; SEMimages of (b) an as-created Au grating by LDW on a 50 nm thick Au film-coatedPET substrate, (c) its 20% strained pattern, and (d) its 30% strained pattern; (e)measured transmittance data of the samples shown in (b–d) along with the bare50 nm thick Au film deposited on a PET piece as a reference (reproduced from ref.40 with permission from the American Chemical Society).

7688 | J. Mater. Chem. C, 2013, 1, 7681–7691

may restrict broader choices of materials and congurations.While eliminating the need for templates or masters as well asworking continuously, a more straightforward and linearlycontrollable patterning method would be called for to addresscertain needs and applications.

Vertical indentation-driven patterning: VIP

We nally introduce a template-free and high-throughputpatterning methodology named Vibrational Indentation-drivenPatterning (VIP)41 which can achieve perfectly straight gratingpatterns with tunable periods at high speed. Fig. 9a depicts theoverall VIP process where the vertically vibrating at tool edgeperiodically indents the line patterns on a linearly movingsubstrate. Here the controlled vibration is generated by oper-ating the high-speed servo motor with a mass eccentricallymounted on the spindle head.

Fig. 9 (a) Schematics of VIP where a vertically vibrating flat tool edge makesperiodic indentations on a moving substrate; grating patterns of uniform periodsof�3 mm fabricated by VIP on (b) a 50 nm thick Au-coated PETand (c) a polyimide(Kapton); (d) �2 mm period 2D grating structure created on a polycarbonatesubstrate by two sequential VIP processes at orthogonal directions; (e) period-tunable chirped grating directly fabricated on a polycarbonate substrate by real-time modulation of the tool vibration frequency during a single VIP process(reproduced from ref. 41 with permission from John Wiley and Sons).




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In VIP, the indentation period l is straightforwardly given asl ¼ V/f, where f and V are the tool vibration frequency andsubstrate moving speed, respectively. Therefore, the resultinggrating period is fully tunable simply by adjusting f and V,without predened masters. Depending on whether f and V areset constant or modulated in real time during the VIP process,the resulting grating period can be uniform (e.g. Fig. 9b and c)or variable (e.g. chirped grating, Fig. 9e). The trench geometrycan also be readily controlled by the indenting force (vibrationamplitude) and the tool tilting angle, as will be demonstratedbelow.

The principle of sequential indentations in VIP enables thecontinuous creation of micro/nano-scale grating patterns overinnitely long exible substrates, by using a nite size atindenting tool. This continuous, mould-free processabilitymakes it practically very appealing to apply VIP to the scalablemanufacturing ofexible templates for large-area R2Rprocesses.

Fig. 10 Fabricationofangle-tunable blazedgrating structuresbyVIP: (a) schematicdrawings of the angle-tunable VIP patterning by adjusting the tool tilting angle; (b)blazedgratingsof variousblazedangles fabricatedonPFAfilmsby angle-tunableVIPprocessing; (c) fabrication and characterization of the wire-grid IR polarizer using aVIP-created blazed grating. On one of the blazed planes of the�1 mmperiod blazedgrating (left inset), a 50nmthickAl layer is shadow-deposited (right inset) toproducea wire-grid polarizer working in the IR range. The transverse magnetic (TM) andtransverse electric (TE) mode transmittances, measured by Fourier-TransformInfrared (FT-IR) spectroscopy, are plotted along with the calculated extinction ratios(i.e. TM/TE) (reproduced from ref. 41 with permission from John Wiley and Sons).


As another purely mechanical method, VIP can make gratingpatterns on any substrate materials soer than a tool as exem-plied in Fig. 9b and c. For instance, the thin metal layer can becleanly cut into the metal grating pattern (Fig. 9b). Even on apolyimide (Kapton) lm which can hardly be patterned viamostof the conventional methods due to its high chemical stabilityand Tg, VIP can directly make clear grating patterns as shownin Fig. 9c.

More sophisticated periodic structures can also be readilyfabricated by multiple VIP processes on the same substrate; forexample, two VIP processes can be sequentially performed rstalong one direction and subsequently in the orthogonal direc-tion to create 2D square mesh patterns (Fig. 9d). This feature isparticularly advantageous beyond other conventional tech-niques as well as the inscribing-based DNI or NCL, since thetool edge does not sweep the substrate surface between inden-tations in well-controlled VIP. More remarkably, by the real-time modulation of the tool vibration frequency and/or thesubstrate feeding rate, the period-variable chirped gratings canbe easily achieved in a single VIP process. Fig. 9e shows the VIP-fabricated chirped grating where the vibration frequency wasvaried under the constant-rate substrate feeding; a similarresult can be obtained by applying the xed-frequency inden-tation over the substrate moving in variable feeding rates.

Since the VIP-created trenches have the V-shaped prolesdue to the indentations of a right-angle cleaved Si edge, theresulting pattern is an intrinsically blazed grating structurewhich is hard to make via conventional micro/nano-fabricationtechniques. Furthermore, VIP enables the one-step fabricationof angle-tunable blazed gratings simply by controlling the tooltilting angle (q) as illustrated in Fig. 10a. Various optoelec-tronics and photonics applications can be derived from suchchirped gratings74,75 or blazed gratings,76,77 to which VIP canprovide an extensive and high-throughput solution; as oneexample, an IR polarizer can be directly fabricated from the VIP-created blazed grating where one of the blazed planes is coatedwith a metal layer (e.g. Al) by shadow deposition (Fig. 10b).


Conclusions

We have reviewed a series of continuous unconventionalnanopatterning methodologies that accomplish higherthroughput and scalability. R2R enables large-area NILpatterning by rolling the exible mould over the continuouslyfed resist-coated substrate. DNI/NCL further realizes seamlessnanograting fabrication with a much reduced area of moulds bydynamically sweeping only a cleaved edge of a mould over thesubstrate. Eliminating the need for prefabrication of masterpatterns, LDW makes use of spontaneous buckling driven by aat edge sweep and VIP uses vertical vibration of a at sharpedge for the cost-effective, template-free nanograting fabrica-tions. These high-throughput continuous nanopatterningtechnologies based on mechanical deformation may open a wayto the practical and scalable nanomanufacturing of various sub-wavelength patterns that could nd a myriad of applications inoptoelectronics, sensing, and energy conversion.

Acknowledgements

The authors gratefully acknowledge the support by the NSFgrants CMMI 0700718 andCMMI 1000425 andNissanChemical.

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