journ oal f materials chemistry - nano-jets · 2013-11-13 · synthetic molecular design and...

ISSN 2050-7526

Materials for optical and electronic devices

Journal ofMaterials Chemistry Cwww.rsc.org/MaterialsC Volume 1 | Number 46 | 14 December 2013 | Pages 7645–7768

FEATURE ARTICLELuana Persano, Andrea Camposeo, Dario PisignanoIntegrated bottom-up and top-down soft lithographies and microfabrication approaches to multifunctional polymers

Themed issue: Fabrication technology of nanomaterials

Integrated bottom-up and top-down soft lithographiesand microfabrication approaches to multifunctionalpolymers

Luana Persano,*ab Andrea Camposeo*ab and Dario Pisignano*ac

Organic multifunctional materials are becoming increasingly relevant to many fields of technology.

Bottom-up assembly processes to give well-defined supramolecular architectures are a powerful tool to

control the size and organization of molecular and polymeric nano- and microstructures. The full

exploitation of these molecular systems in devices, however, often requires a superior technological

control which should span very different length scales, enabling nanoscale control as well as large-area

patterning and interfacing of active molecules with electrodes, external optical excitation or collection,

biotechnological and lab-on-chip platforms of practical use, and so on. This critically depends on the

development of specific lithographic approaches which are inherently hybrid in their character, since

they combine bottom-up and top-down nanofabrication strategies in a smart way. Here we review and

discuss some of these relevant hybrid methods, with a focus on their ultimate applicability to device

platforms. The invention of a successfully integrated bottom-up/top-down strategy for microfabrication

of functional macromolecules relies on exploiting their peculiar physico-chemical properties, and on

building genuinely cross-disciplinary know-how and technologies.

1 Introduction

In the last two decades, micro- and nanofabrication technolo-gies have denitely moved away from simply being tools topattern inorganic materials for optics or electronics. Instead,these technologies are becoming more and more powerful in

Dr Luana Persano, PhD inInnovative Materials and Tech-nologies, is currently aresearcher at the NationalResearch Council-NanoscienceInstitute. She has been a Marie-Curie fellow at FORTH, Greece,and visiting scientist at HarvardUniversity and the University ofIllinois. Her research interestsinclude nanomanufacturing andlithographic processes onorganics and nanocomposite

semiconductors, nanophotonic and piezoelectric devices, andelectrospinning technology transfer. She has authored about 65papers on international journals and one international patent.Among other prizes, she received the “CNR-Start-Cup” award in2010 and the “Bellisario” award as Young Talent in IndustrialEngineering in 2011.

Dr Andrea Camposeo, PhD inPhysics (University of Pisa), is astaff researcher at the Nano-science Institute of the CNR(National NanotechnologyLaboratory of Lecce). He hasbeen a visiting scientist at theUniversity of Bonn, Toronto,Harvard and FORTH and hasauthored more than 70 papers.His research activities includethe investigation of the opticalproperties of light-emitting

polymers, composites, organic crystals and polymer nanobers,the design and characterization of organic-based light-emittingdevices and the development of optically active nanostructures byelectrospinning and two-photon lithography for nanophotonicapplications.

aNational Nanotechnology Laboratory of Istituto Nanoscienze-CNR, via Arnesano,

I-73100 Lecce, Italy. E-mail: [email protected]; Andrea.camposeo@nano.

cnr.itbSo Materials and Technologies S.r.l., Via Leuca, I-73020, Cavallino (LE), ItalycDipartimento di Matematica e Fisica “Ennio De Giorgi”, Universita del Salento, via

Arnesano, I-73100 Lecce, Italy. E-mail: [email protected]

Cite this: J. Mater. Chem. C, 2013, 1,7663

Received 24th May 2013Accepted 20th September 2013

DOI: 10.1039/c3tc30978a

www.rsc.org/MaterialsC

This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. C, 2013, 1, 7663–7680 | 7663

Journal ofMaterials Chemistry C

FEATURE ARTICLE

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generating and shaping layers, nano- or micro–nanostructures,made of polymers or of other molecular systems. Such adevelopment has consequently greatly widened the range ofapplications enabled by lithographic techniques. In thisframework, functional and so-called multifunctional poly-mers,1–4 namely organic macromolecules in which two or morefavorable structural, optical, electronic, or biological propertiesare concomitantly present, are playing a pivotal role in moti-vating the development of advanced, specic lithographic tech-niques. Active polymeric patterns with feature resolution fromthe micro- to the nanoscale allow researchers to exploit both theenormous variety of chemical functionalities achievable bysynthetic molecular design and biological routes, and theusefulness of structurally stable, patterned features embeddablein novel materials and devices. The resulting combination isextremely attractive for photovoltaic and light-emitting devices,chemical and biological sensors, energy harvesting systemsbased on piezoelectric organics, protective coatings and smartsurfaces, hybrid biological andmicrouidic chips, nanomedicineand tissue engineering applications. In addition, the possibilityto blend these polymers with semiconducting or carbon-basednanostructures, including carbon nanotubes, nanobers andgraphene, further enhances existing structures and devices.

The realization of multifunctional polymer nanostructures,as well as their embedment into device platforms, can becarried out by either top-down or bottom-up methods. On onehand, the top-down concept has been driving miniaturization fordecades. This approach is based on the realization of micro-structures, nanostructures and devices following the reductionof the size of blocks of materials having larger dimensions.Therefore, the spatial resolution, namely the minimum featuresize which can be obtained, depends mainly on the accuracy ofthe material removal. Such accuracy is in turn related to a largeset of variables including especially temperature, used chem-icals, and all the physical effects characterizing the interactionbetween somatter and radiation. In addition, the capability tocontrol and reduce diffraction in radiation beams used toexpose materials oen plays a key role, as recalled in thefollowing. Instead, the bottom-up concept is inherently additive

and involves the spontaneous or induced assembly of funda-mental, nanoscale components into more complex architectures.In this framework, a special role is played by self-assemblyprocesses, namely by the spontaneous organization of basicelements, including molecular components, into larger struc-tures and systems. The assembly is driven by energy minimi-zation and entropy maximization mechanisms, and requiressome degree of adjustability of single building blocks.5 Forthese reasons, the assembly can be favored by thermal orexternal agitation. Furthermore, in many of these processes,uids are important. They can favor the self-organization ofassembling components through capillarity phenomena medi-ated by solid–liquid interfacial energies, solvent evaporation,etc. Involving directly molecular or even atomic constituents,the resolution achievable in principle by bottom-up lithogra-phies is only limited by the granular nature of matter. However,reproducibility, throughput, and large-area processing arealmost ubiquitously relevant issues.

When dealing with inorganic materials, the top-down andbottom-up concepts are generally well distinct, and differentlithographic approaches can be effectively categorized into thetwo classes of methods. Basically, top-down techniques includeexposure-based approaches, in which a polymeric resist ispatterned by virtue of the variation of its solubility due to theinteraction with specic radiation. Such radiation delivers asuitable amount of energy to the material. In turn, this energyallows the organic compound to either cross-link or decompose,depending on its specic formulation. The high resolution andthe registration capability are important points of strength ofthese approaches, allowing overlay patterning. Exposure-basedlithographies include single-photon optical lithography(schematized in Fig. 1a), multi-photon methods, holography,electron-beam lithography (EBL), X-ray lithography, etc. Theradiation can be delivered either in parallel onto large areas orby serial writing, depending on individual technologies. Inparticular, the broad domain of photolithography includesmany sub-techniques such as phase-shi, projection, andextreme-UV lithography. The achievable resolution is at thescale of, say, a few tens of nm and of a few nm for opticallithographies and for EBL, respectively. However, these gurescan be obtained only with extremely controlled and optimizedprocesses, and with highly expensive equipment. In most of theresearch laboratories, the minimum feature size which isroutinely obtained is larger than the best performances repor-ted above, by at least one order of magnitude. Following expo-sure, the development of the exposed resist in a suitable solventremoves selectively the polymer in the irradiated (for positiveresists) or in the non-irradiated (for negative resists) areas.Then, wet or reactive ion etching or milling processes are usedto transfer the pattern onto the underlying inorganic material.As an (costly) alternative, focused-ion beams can also beemployed for direct milling on various materials. Anotherpossibility of exposure-based techniques, enabled by carefulchemical design, is to pattern directly active organic materials.This would allow one to dene regions with specic functions,as demonstrated by photo-patterned, multicolor uorene-basedpolymers exhibiting electroluminescence (Fig. 1b).6

Prof. Dario Pisignano, PhD inPhysics, is an Associate Professorat the University of Salento, andcoordinates an interdisciplinarygroup working on Nanotechnol-ogies on So Matter at theNational Nanotechnology Labo-ratory (CNR-Nano). He hasauthored about 170 papers andve patents. His research activityis focused on the development ofadvanced polymer processingand lithographic approaches,

electrospinning, and so lithographies, for realizing micro-nano-structured devices and functional systems. He is the recipient of theNANO-JETS ERC Ideas Starting Grant.

7664 | J. Mater. Chem. C, 2013, 1, 7663–7680 This journal is ª The Royal Society of Chemistry 2013

Journal of Materials Chemistry C Feature Article


Bottom-up methods work at the nm- or even sub-nm scaleand include the formation of colloidal nanocrystals and ofepitaxially grown quantum dots and nanowires through self-assembly, which are important examples in nanochemistry andoptoelectronics. Some approaches have been proposed, whichrely on complementary lithographic strategies in order torealize devices with these nanostructures and nanoparticles.How the top-down and the bottom-up paradigms can becombined in a synergic way to fabricate eld-effect transistorsbased on inorganic semiconductor nanowires has been recentlythe subject of a dedicated review.7

In general, the fabrication techniques suited to patternfunctional polymers can lead to a more in-depth integration ofdifferent paradigms. These methods are oen unconventionaland largely hybrid in their character.8 They can show thecapability to tightly control the formation of micro- and nano-structures by the choice of a well-dened set of processparameters (which is typical of top-down technologies), and touse molecular self-assembling processes and weak interactions(typical of so matter and more in general of bottom-upapproaches). So lithographies,9 nanoimprinting,10 patterningmethods based on block copolymers (BCPs),11 and fabricationof polymer nanobers by electrospinning,12 schematized inFig. 1c–f, are naturally at the edge between top-down andbottom-up. Many of these techniques allow various molecularassemblies or eventual molecular recognition steps to beincorporated during nanofabrication. In all these techniques,resolution can be pushed below 10 nm. Many active organicmaterials benet from being soluble in commonly usedsolvents, and their complex assembly behavior during dryingcan be driven by so lithographic templates or by electried jets

leading to solid nano- or microstructures. Following solidica-tion, the resulting features are structurally stable. This is due tothe lack of transversal diffusion, which is related to theconsiderable size of macromolecules, and to the goodmechanical properties, which are related to the favorableviscoelastic properties and glass transition behavior of manypolymeric materials.

It is also clear that the integration of functional polymersinto working devices for optoelectronics, nanoelectronics,microuidics and biotechnology has to pass through one ormore top-down fabrication steps. These are critical to coupleorganic micro- or nanostructures with further functional layers,metal electrodes, scaffolds for structural support, etc., or to givepolymers new or enhanced functions. Interfacing patternedpolymer structures to electrodes and other device featuresneeds the combination of lithographic methods specialized fororganic active materials and of more conventional, top-downfabrication strategies.

For all these reasons, research on integrated patterningapproaches, applied to functional polymers and to deviceplatforms based on them, is nowadays vibrant. This featurearticle comprises ve sections. In the next section, we presentan overview of some remarkable lithographic and nano–microfabrication techniques. When applied to active organicpolymers, these methods have a clear added value withrespect to conventional technologies. Then, we move toconsider integrated bottom-up and top-down methods, andhow these can open new directions in terms of device fabri-cation. Finally, we draw some conclusions together with anoutlook of these hybrid and extraordinarily promisingconcepts.

Fig. 1 Schemes of different microfabrication technologies which can be performed on functional polymers. (a) Optical lithography (irradiation through an optically orUV-transparent photomask, development of the cured polymer). (b) A multicolor organic light-emitting device realized by photolithography on photosensitive,crosslinkable and electroluminescent oxetane-functionalized spirobifluorene-co-fluorene polymers emitting blue, green and red light. The photographed glasssubstrate has an area of 25 � 25 mm2. Adapted with permission from ref. 6, Nature, 2003, 421, 829, DOI: 10.1038/nature01390. Copyright ª 2003, MacmillanPublishers Ltd. (c) Soft lithography (replica molding process, and subsequent conformal contact of the produced elastomeric mold onto a functional polymer layer. Softmolding, allowing a pattern to be realized on the active polymer by heating above the glass transition, is a possible process enabled by such conformal contact). (d)Nanoimprinting. Pressing can be carried out at either high temperatures or at room temperature. (e) Example of process based on BCP domain separation, driven byunderlying self-assembled monolayers. The grey features indicate a preliminarily prepared pattern (for instance made of a photoresist) used to obtain a chemicallyanisotropic substrate, in turn used to direct phase separation in BCPs. (f) Electrospinning.


Feature Article Journal of Materials Chemistry C


2 Micro–nanofabrication techniques fororganic materials

Approaches to pattern functional polymeric materials need tomeet a few crucial requisites in order to not deteriorate thephysico-chemical properties of organics:

(i) Patterning should be preferably carried out at roomtemperature and without the use of high-energy radiation orother factors promoting oxidation.

(ii) Flexibility is oen important, meaning the capability ofpatterning and nanofabrication technologies to be used with asmuch polymers as possible, and, whenever possible, ofproducing exible nanostructures and devices whose mechan-ical properties can be tailored for specic applications.

(iii) Last but not least, operational simplicity and cheapness,which are always desired in lithographic technologies, areespecially sound for methods targeting molecular compounds.In fact, these are key factors determining how much eachtechnique is actually accessible by low cost bio-organiclaboratories.

2.1 So lithography

So lithography was invented by Whitesides and coworkers inthe early 1990s.13–15 This set of techniques has been the mostsuccessful to pattern functional organic materials. Severaloutstanding reviews already exist in the literature,8,9 thereforewe limit here to summarize the main aspects making solithographies so exible and easy to integrate with comple-mentary lithographic procedures. The fundamental character-istic of so lithography is the use of elastomeric molds made ofpolydimethylsiloxane (PDMS), of its hard variant obtained fromtrimethylsiloxy-terminated vinylmethylsiloxane–dimethylsilox-ane and methylhydrosiloxane–dimethylsiloxane copolymers,16

of photocurable peruoropolyether,17 or of other polymers.These molds serve as basic elements to dene and transferpatterns onto the ultimate target materials. The elastomericmolds are produced by replica molding,9,14 namely by in situpolymerization of the initially pre-polymeric, uid elastomericcompound against the features of a master structure (Fig. 1c). Inturn, the masters are to be generated by other, oen conven-tional lithographic methods, hence so lithography has to beconsidered as complementary to (and largely enhancing) stan-dard patterning approaches more than a strategy to replacethem. The replica molding is the key enabling technology at thebase of the so lithography approach, and how much solithographies can be applied to functional polymers anddifferent solvents critically depends on the elastomer used toproduce replicas. Aerwards, themolds carrying a negative copyof the master features are peeled off and are placed inconformal contact with a substrate. The pattern transfer istherefore performed by a mechanical contact without the needfor radiation beams and potentially harsh chemicals as thoseused in most of the etching processes. Therefore so heremeans gentle, chemically exible, as well as suitable to beapplied to so matter, including functional polymers. Theultimate resolutions are not limited by beam diffraction as in

optical lithography, but instead by the size and conformation ofmacromolecules at the nanoscale, and by pattern distortionspotentially occurring during the process due to the so char-acter of both elastomeric elements and polymers to bepatterned. With properly optimized conditions and parameters,features with transversal and vertical dimensions of a few nm,or even sub-nm,18 can be obtained, fully in line with valuesachievable with much more expensive, low-throughput high-resolution writing techniques such as EBL. In fact, while theserial, lithographic production of suitable masters presentingfeatures at the nm- or sub-nm scale is very demanding, theenormous potentialities of replica molding in terms of achiev-able resolution have been better demonstrated by transferringinto elastomers' alternative master structures derived frombiology or chemistry, such as ionic crystals showing verticalsteps of 0.3–0.5 nm,18 single-walled carbon nanotubes of�1 nmdiameter,19,20 micelles of various shapes, viruses,20 etc. Thespatial uniformity of the so-produced features is at the sub-nmlevel as well. Indeed, the other point of strength which deservesto be mentioned is the extreme cheapness of these approaches,making so lithographies fully accessible to organic chemistryand biotechnology laboratories. At least in some cases, solithography is demonstrated to be compatible with overlayregistration, thus enabling multilevel fabrication of complexdevices with accuracy reaching a few hundreds of nm forsufficiently rigid elastomeric elements21 and exhibiting highpotential integration with techniques using rigid photomasks.

Whether a specic so lithographic technique has to beconsidered prevalently as a top-down or a bottom-up methodlargely depends on the details of the nal pattern transfer step,which can be very diverse. Micromolding in capillaries (MIMIC),namely microuidic lithography,13 and even nanouidics can beimplemented by using elastomeric elements as templates todene micro- or nanochannels, and applied to pattern manyactive polymers including conjugated compounds to realizeorganic waveguides, channels for eld-effect transistors, light-emitting nanobers and so on.22 In these approaches, polymericsolutions or prepolymers are let to ow into elastomeric chan-nels due to spontaneous capillarity or external pumping, andsolid features are obtained following solvent evaporationthrough the molds or from the open channel terminations, orupon curing. Micro- and nanouidic lithographies are thereforeinherently additive methods, generally performed entirely atroom temperature, and very suitable to pattern active materialsgiven that the used solvents are compatible with the elastomersconstituting the molds. As an alternative, various solvent-assisted methods assisted by elastomeric molds have beendeveloped in the last two decades,15 which may be of particularinterest in combination with the use of conjugated polymers torealize active electronic or photonic components includingdistributed feedback lasers. Microcontact printing (mCP) isanother well-established method, in which the patternedsurface of the mold is inked by molecules and placed inconformal contact with the target substrate.23 In this way,molecules are transferred onto the target only when this is incontact with the protruding features of the mold, and when thesubstrate–ink interactions are stronger than the attractive




forces between the ink molecules and the elastomer. The clas-sical working scheme of mCP is therefore based on a covalentsubstrate–ink interaction, assisted by non-covalent, van derWaals or electrostatic links organizing the transferred mole-cules in ordered monolayers. In many other cases, the inktransfer can be primed by electrostatic forces as well, andprinted molecules are just physically adsorbed on the targetsubstrate. While being much more easily applied to moleculeswith low molar mass rather than to polymers, mCP is quiterelevant in terms of bottom-up and top-down integration. Inparticular, involving so closely molecular self-assembly, mCP isthe so lithographic method which more effectively builds onthe bottom-up concept.

2.2 Nanoimprint lithography

Nanoimprint lithography (NIL), proposed by Chou in 1996,10 is apowerful top-down technology. Similar to so lithographies, itdoes not use radiation beams to expose resist materials, but it isbased instead on the mechanical contact between a targetorganic material and a mold. The molds can be either inorganic(metallic, semiconductor, etc.) or organic (highly crosslinked orhigh-glass transition temperature polymers). Importantly, theinitial pattern is not exploited as a master for realizing second-generation patterns in elastomers as in replica molding. Incontrast, the mold is directly used to imprint the nal targetmaterials upon application of pressure and following heating(Fig. 1d). In conventional NIL, heating is needed to drive thetarget material, which is a thermoplastic polymer, above itsglass transition temperature, Tg. As known from the physics ofthe glassy state, this corresponds to lowering the polymerviscosity (or, in other words, its characteristic structural relax-ation time) by many orders of magnitude, thus allowing theorganic material to ow under the applied pressure and to llthe recessed feature of the imprinting mold.

From the physical viewpoint, NIL is described by theconcomitant effect of at least two mechanisms. On one side, thepattern transfer is strictly related to the mechanical response ofamorphous polymers to the external pressure. This response isinuenced by the large conformational exibility of individualpolymer chains as well as by the presence of topologicalnetworks of entangled chains.24 A higher conformational exi-bility is related to local molecular motions, whereas long-range,structural motions generally need the weakening of entangle-ments. Following the initial increase of temperature, the poly-mer lm can reach its so-called rubber-elastic region, where thetime-dependent shear compliance, J, shows a plateau. At thisstage the transferred deformation is still reversible, whereas athigher temperature or for longer heating times, irreversiblepatterning is obtained leading the polymer to its terminal owregion. Here J has a linear dependence on time. Anothermechanism favouring imprinting involves pressure-inducedvolumetric contraction. A certain amount of free volume, nf, isalways present at the nanoscale in deposited organic layers. nf isroughly given by n � n0, where n and n0 indicate the specicmolecular volume and the characteristic incompressiblevolume (hard core) of molecules, respectively. The reduction of

nf due to the application of an external stress helps the pressedlm in taking the shape of the mold.25 The ultimate resolutionachievable by NIL is of the order of a few nm, limited by thegranular size of macromolecular chains and networks.

All these physical considerations apply to thermoplasticpolymers, but some can hold for oligomers and conjugatedpolymers as well. These materials generally show a quite poormechanical response due to the low molar mass and to thereduced amount of entanglements, therefore they are verydifficult or impossible to pattern at high temperatures. In fact,NIL has been developed to work at room temperature (i.e.,without leading polymers above Tg) and on a much largervariety of organic materials including conjugated polymers. Dueto the slower response at room temperature, imprinting timesare oen longer than those used in conventional NIL.Notwithstanding the consequently reduced throughput, thepeculiar advantage of room temperature NIL is the much higherchemical exibility. In particular, the method is especiallysuitable for patterning functional polymers, since avoidingheating allows us to carry out the process in an uncontrolledatmosphere even with delicate materials which are easilysusceptible to oxidation. Working at low temperature alsomeans reduced deterioration of the pressing set-up togetherwith a higher potentiality of use in conjunction with precisionmechanics and optics for overlay alignment. For these reasons,room temperature NIL has been applied also in several experi-ments demonstrating a high degree of integration with otherlithographic approaches. Finally, different physical and chem-ical mechanisms can be exploited to assist pattern formationduring nanoimprinting processes besides viscoelasticity,including thermally driven or especially UV-driven cross-linkingas in step-and-ash methods,26 and photothermal molding asdemonstrated for the ultra-fast patterning of semiconductors27

and nanocomposites.28 Overall, similar to so lithographies,the resolution achievable by NIL methods can be pushed at thenm scale, and three-dimensional patterns and architectures canbe fabricated by various strategies, including the developmentof multilevel processes29 and of imprinting inclined withrespect to the horizontal direction.30

2.3 Block-copolymer lithography

A BCP is a macromolecule in which two or more chemicallydifferent homopolymer units (polyA, polyB, etc.), each made ofmany monomers (A, B, .), are linked together. Each of theseunits is a block, and the simultaneous presence of different,covalently bound blocks in the molecule (polyA-b-polyB) leadsto peculiar chemico-physical properties of the overall system. Inparticular, lithographic approaches based on BCPs are based onthe capability of self-assembling of surface-depositedcopolymer macromolecules, which can produce highly periodicand oriented, chemically anisotropic domains with extremelyhigh feature resolution due to phase segregation (Fig. 1e).11,31–34

Indeed, the underlying physical mechanism at the base of theseself-assembly processes is the aggregation of the chemicallydifferent blocks in spatially separated and dense domains,because of the poor relative affinity of the different monomer




species (i.e. because of the mutual immiscibility of the differentcomponents). Various topographies can be obtained in thisway, including spheres, cylinders, lamellae, and gyroids, bycontrolling molecular parameters such as the length of themolecular chains and the volume fraction of each blockcomponent, and the thermodynamics at the base of the phasesegregation process by which the system seeks to minimize itsfree energy has widely fascinated materials scientists.35 Forexample, the blocks can be described by the Flory–Hugginsparameter (c) associated with the mutual interaction betweenthe different constituent monomer species. c is directly relatedto the increase of energy due to a possible intimate mixing ofthe different species at the molecular scale. Hence, it is clearthat blocks whose monomers show high c values will prefer-entially segregate into well-separated and chemically well-dened domains, due to the disfavored contact between thedifferent monomer species.11,35 The resolution achievable inthis self-organization is of the order of a few nm, which is muchbetter than the resolution of features routinely obtained inoptical lithography, the lateral uniformity of the features is inprinciple at the sub-nm scale, and BCP lms, which can bedeposited onto large areas by simple solution-based methodssuch as spin-coating, generally exhibit a remarkable mechanicalstability. A lot of research has been focused on obtaining thehighest possible degree of order (possibly in long ranges) inalternating the different domains in BCP lms, on reducingthe amount of defects at the separating interfaces, and oncontrolling the orientation of the resulting nanostructures.Indeed, as in other assembly processes, interfacial interactionswith the substrate also play a crucial role, and the organizationof copolymer blocks can be templated by previously realizedchemical or topological patterns, spatial connement or byexternal elds or shears helping in realizing specic desiredgeometries.32,36,37 In particular, pre-existing chemical patternswith regions of different surface energies can strongly favor thepreferential wetting of a specic region by a particular compo-nent of the BCP, thus enhancing and controlling the phasesegregation in the deposited copolymer lm. Another importantissue is making the BCP self-assembly steps more and morecompatible with other, well-established fabrication routes andwith the geometries which are interesting for electronic andphotonic devices. Fortunately, a proper process optimizationallows features to be formed, which not only show highlycontrolled sizes and shapes but also differ substantially fromthose obtained under bulk conditions. For instance, the BCPlithography has been recently developed to produce nanoscalesquare patterns which are quite suitable for integrated circuitmanufacturing.34 Arrays of lines with sharp bends, modelingpatterns of interest for the production of microelectronicdevices, can also be fabricated by using blends of BCPs onchemically patterned substrates.37 Pre-existing topographiesused to direct self-assembly can also help in enhancing theregistration capability of BCP lithographies, facilitating theirapplication in multi-level processes for the realization ofcomplex devices, and so on.

Once the BCP pattern is obtained, it can be used for a widevariety of further lithographic or micro-nanofabrication steps,

such as the selective decoration of given domain species withnanoparticles, the fabrication of active polymer nanobers andof nanoporous membranes, and the selective etching of theunderlying substrate to transfer features to another organic orinorganic layer following the removal of a particular domain.Being so versatile, BCP lithographies are among the most suit-able methods allowing top-down and bottom-up processingconcepts to be mixed together, as reviewed in several examplesin the next section. Finally, it is worth mentioning that, due totheir exibility, as well as to their cheapness and simplicity,these techniques are now moving from laboratories to indus-trial applications.

2.4 Electrospinning

Electrospinning allows the formation of polymer nanobers byan electried jet of solution, in which molecular entanglementsare sufficient to provide the spun body of uid with viscoelasticbehaviour.12,38,39 Being inherently based on weak intermolecularinteractions and allowing formation of ultra-thin polymer la-ments, thus shaping so matter in an additive fashion throughsolution processing, electrospinning could be broadly catego-rized as bottom-up technology. The chemical exibility of themethod is quite high though not complete, because electro-spinning is very suitable to process a wide range of plasticpolymers and of their blends with other soluble molecules(dyes, soluble oligomers, proteins and enzymes) and withnanoparticles and micro-objects including living cells. In thisprocess, a polymer solution is generally placed into a syringe,and an electric voltage bias (V, in the range of 2–100 kV, mostoen around 10–30 kV) is applied between the syringe tip and acollecting surface by means of a high-voltage generator (Fig. 1f).Compared with other deposition methods from solution, theconcentration of the polymer in the used solvent in electro-spinning is generally high (5–30% in weight), thus favoringentanglements and consequently improving the viscoelasticproperties of the jet. Firstly, a pendant droplet is found at thesyringe tip, and an excess charge accumulates at the liquid–airinterface due to the applied eld. Such a droplet is progressivelydeformed following the increase of the voltage bias, thusforming a so-called Taylor cone, until electrostatic forces over-come the solution surface tension and a jet is extruded andmoves from the Taylor cone to the collectors. The jet velocity isof the order of 1 m s�1 and the jet accelerations are of hundredsof m s�2, corresponding to deformation rates which can reach106 to 107 s�1. During the ight of the polymer solution jet fromthe spinneret to the collecting surface, the stretching ofmacromolecules is therefore relevant, and the solvent evapo-rates concomitantly which leads to the very rapid narrowing ofthe uid body from the order of 100 mm (corresponding to theinternal diameter of the syringe tip) to potentially less than10 nm. Most of the reduction of the jet transversal size seems tooccur during a few instability stages, setting along the trajectoryof the jet which is rectilinear only in the rst part. The interplayof jet instabilities, solvent evaporation, and possibly phase-separation phenomena driving the assembly of macromole-cules lead to a complex scenario and to the realization of




continuous nanobers which may have very different porositiesand morphologies. As studied by Rutledge and coworkers,40

comparing the drying time (tD), the so-called buckling time (tB),and the characteristic phase separation time (tPS) allows one topredict the resulting features of collected nanobers. tD and tBare dened as:

tD ¼ v0

AWEV

; (1)

tB ¼ D�fp � fp0

�2

WEV2

; (2)

where v0 is the initial liquid volume, A is the initial vapor-dropsurface area, WEV is the initial evaporation rate, D is thepolymer–solvent mutual diffusion coefficient, and fp and fp0

are the polymer fractions at the external skin which formsduring the jet ight and at the uid core, respectively. Forinstance, solid or porous bers are obtained depending onwhether the ratio, tPS/tD, is greater than or less than 1, respec-tively. The buckling time affects instead the surfacemorphology. Indeed, solid bers can be buckled (if tD/tB > 1) orsmooth (tD/tB < 1). Analogously, porous bers can be buckled(tPS/tB > 1) or smooth (tPS/tB < 1).40

Overall, the complexity of the process is increased by thehigh number of experimental parameters, which include,beside the applied bias, the polymer solution concentration andthe spinneret–collector distance (usually from several cm to afew tens of cm), a large set of other properties of the usedsolutions (polymer molecular weight, solution density,conductivity, viscosity, characteristic relaxation time anddielectric constant), of the specic used set-up such as thesolution ow rate generally applied in the syringe by means ofexternal pumps, and the environment in which bers are real-ized, such as temperature, relative humidity, etc. Operationally,the process is instead simple and cheap, and easily scalable tothe pre-industrial level,41 which explains in part its extraordi-nary success. What is more relevant in the present framework,however, is that electrospinning has been widely engineered inthe last decade, in order to become, on one side, more andmorecompatible with conjugated and other functional polymers,which rarely show favorable viscoelastic properties and conse-quently are oen difficult to electrospin directly, and, on theother side, integrated with other fabrication technologies andwith a larger and larger variety of usable substrates, pre-patterned lithographic features, devices, and applications. Forinstance, varying the geometry of the collecting surfaces or theirdistance and movement relative to the spinneret is a direct wayto control the position and assembly of nanostructures andenhance their integration in devices, or to achieve multifunc-tional polymer nanobers. This is the case of near-eld electro-spinning invented at the Universities of California at Berkeleyand of Xiamen.42 In this method the spinneret–collectordistance is kept well below 1 cm, jet instabilities are avoided orgreatly reduced, and motorized scanning tips or collectors areused to directly draw polymer nanobers. Other advantagesinclude the possibility to reach very high values of the electricelds (107 V m�1), to use moderate bias (below 1 kV), and to

enhance the stretching of polymer molecules and their orien-tation within the formed bers. Near-eld electrospinning hasbeen applied to thermoplastic polymers such as polyethyleneoxide,42 to conductive polymers such as polyaniline, and topiezoelectric polymers such as poly(vinylidene uoride)(PVDF).43 Though generally ineffective to pattern large areas dueto its serial character, this approach allows electrospinning tobe used as an additive patterning technique of individualnanostructures, with extremely high positioning precision andsub-100 nm feature resolution,44 and with no need for maskingor high-energy exposures. Different mechanisms such as pyro-electrodynamic shooting have also been demonstrated to be veryeffective for depositing individual nanodroplets, based onpyroelectric forces.45 Once deposited, individual polymernanostructures can in turn enable other lithographicapproaches, used as templates for shaping elastomericelements for so lithographies, as masking features for etchingand li off processes, or as functional elements per se, includinglight-emitting or piezoelectric laments or droplets. Finally, thenanopottery approach is another example of how electro-spinning can be developed, focusing the electried jet on sharpelectrode tips and forming free-standing, hollow coiledmicrostructures.46

3 Bottom-up and top-down integration

The combination of so lithographies based on molecular self-assembly with top-down nanofabrication methods has beenproposed since the invention of mCP.23 mCP is an additivetechnique, which is an intrinsic merit since this allows thewaste of material to be minimized. However microcontact-printed self-assembled layers can also be used as resists withnanoscale thickness in order to remove unprotected regions ofthe underlying substrate by means of selective chemicaletching. This approach is nowadays well-established, and it hasbeen used to realize many different devices both by inorganicand by organic functional materials. Furthermore, PDMSstamps can be accommodated by conformal contact on cylin-ders rolling over the target substrate, thus allowing the overallmCP throughput to be greatly improved.47 This concept ofsomehow making mCP dynamic also leads to using stampsmoving or jumping during the process of fabricating polymer–brush microstructures.48 Some recent studies developed furtherthe integration between so lithographies and complementarypatterning technologies, trying to go beyond the so-called “onetemplate-one pattern” paradigm, which has been at the base ofconventional methods based on replica molding. In fact, instandard so lithography each particular pattern requires thededicated design and fabrication of a specic master structure.This could be a bottleneck especially when working at relativelyhigh resolutions, or when photolithography or EBL facilities arenot promptly available. A possibility to tackle this issue isopened by PDMS-coated dip pen nanolithography49 stamp tips.The resulting approach50 is serial and has several advantages. Itis a highly accurate and controlled variant of mCP in whichmolecular inks are delivered on surfaces by scanning probes,with sub-100 nm resolution and patterning speed up to the




order of 1 mm s�1. Another recently proposed method useselastomeric polymeric microspheres, which can be fabricatedwith low cost and high throughput by means of microuidicchips working with a continuous phase of water and surfactantand a dispersed phase of liquid PDMS.51 Each microsphere,once generated in the microuidic device, is polymerizedthermally, inked by molecular solutions, and then used for thecontinuous delivery of the ink to a substrate similar to a ball-point pen. In this way no master structure is needed (except forthose allowing the original microuidic devices to be fabri-cated), each kind of pattern can be transferred onto the targetsubstrate with real-time control and speeds of many tens ofmm s�1, and the transversal size of each feature can be varied tosome extent through the mechanical compression of the rollingmicroparticle.

A different strategy involves the development of a photo-lithographic approach instead of replica molding to patternPDMS elastomers. In principle these methods allow one toavoid realizing dedicated master structures, thus making theoverall lithographic procedures easier, cheaper and more ex-ible. This can be done, for instance, by employing benzophe-none as an additive to initiate the photo-polymerization ofPDMS,52 or by exposing the already cured elastomer to UV-lithography and then transferring the features into PDMS bychemical development with a 1 : 1 (v/v) mixture of 1 M aqueoussodium hydroxide and ethanol.53 To dene the optical patternboth conventional optical masks and organic features realized,for instance, by microuidics, can be exploited, and theresulting, photolithographically patterned PDMS can in turn bethen used for second-generation pattern transfer in various solithographies including replica molding and NIL-like soembossing, thus demonstrating an intimate integration ofoptical and mechanical lithographies.

In addition, NIL can be combined with a variety of inherentlybottom-up processes working at the nm- or sub-nm scale,particularly molecular imprinting (MIP) which can be exploitedto generate specic sites for molecular recognition in polymericpatterns.54 In this approach, binding sites are obtained in cross-linked polymers by a molecular template such as (S)-propran-olol, via free radical polymerization. Following the removal ofthe molecular template, the resulting sites preferentially bindthe original template over its antipode compound [i.e., (R)-propranolol]. Clearly, superimposing patterns (say, at themicroscale or at higher resolution) to the molecular recognitionsites working at the scale of A, thus producing patterned lms ofmolecularly imprinted polymers, would open interestingperspectives to realize surfaces with enhanced and specicbinding properties. Applications may be in the elds of chem-ical separation and sensing. Reactive NIL is especially useful inthis respect, since it allows patterns of such molecularlyimprinted polymers to be obtained through UV or thermalpolymerization with 100 nm spatial resolution (Fig. 2),54 whichis much higher compared to previously explored methods.Together with such high spatial resolution, the advantages ofthe MIP–NIL combination over other approaches to preparefunctional polymer patterns include the process cheapness andhigh throughput, as well as the high retained molecular

selectivity. In this framework, innovative approaches involvingso lithographies have also been proposed. For instance,polymethylmethacrylate (PMMA) scaffolds realized from PDMSpatterned molds have been imprinted with gelatin or even withwhole cells in order to enhance subsequent cellular adhesion.55

Similar combinations can be gured out which involve conju-gated and multifunctional polymers as well, together withimprinted biomolecules.

BCP assembly is very well suited to be combined with etchingof the underlying substrate material once one of the chemicallyanisotropic components has been selectively removed. Thisallows other functional organic materials to be processed, aswell as inorganic semiconductors, metals etc. The so-calledAirgap technology developed by IBM56 has been an importantmilestone for the integration of bottom-up and top-downmethods, combining standard complementary metal–oxide–semiconductor fabrication with nanopatterning based on self-organization in polymeric layers, at the industrial scale. Herethe metal wires of an electronic chip are mutually isolated bymeans of gaps, realized by using a nanoporous self-assembledpolymer lm. This promises a signicant increase of the chipspeed or equivalently a reduction of energy consumption. Fromthe point of view of lithographic manufacturing, this approachcapitalizes on the merits of polymer self-assembly, such as theoperational simplicity, the low cost, and the possibility topattern large areas with highly dense features. Three-dimen-sional nanostructures can also be produced by BCP thin lms,starting from the cross-linking of cylinder-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) with embeddedreactive –N3 groups.57 This process passes through a complexassembly phenomenon, in which lamellar-forming BCPs ontop of an underlying layer of cylinder-forming material leadsto microdomains oriented normally with respect to the lmsurface, and through the removal of the PMMA component(Fig. 3a–d). These nanostructures are also able to enhance thephoton extraction efficiency of devices typically produced byconventional growth and fabrication approaches, such as light-emitting diodes.

Among other methods for so-called templated or directed self-assembly of BCPs,58 so-graphoepitaxy uses BCPs and opticallithography together, being very promising in terms of low costand ultralarge area patterning.59 The idea here is to fully exploit

Fig. 2 Example of a hybrid lithographic approach combining NIL and MIP. (a) Amonomer mixture (MIP-resist), which contains a suitable molecular template, isdeposited on a patterned NIL stamp. (b) A substrate is pressed against the stamp,and monomers in the MIP-resist are concomitantly polymerized thermally or byUV radiation. (c) The stamp release leads to a MIP pattern with molecularlyimprinted binding sites at the nanoscale. Reproduced with permission from ref.54, Analyst, 2010, 135, 1219, DOI: 10.1039/c0an00132e. Copyright ª 2010, TheRoyal Society of Chemistry.




the lateral connement provided by pre-fabricated photolitho-graphic micropatterns, to direct and orient the lamellar nano-patterns in BCPs as shown in Fig. 3e–g.59 These can beultimately transferred to obtain various functional nano-structures made of different materials. Due to the integration ofphotolithography and BCP assembly, this method is especiallyversatile in terms of the possibly produced structures, and canbe scaled up easily to pattern arbitrary areas with featureshaving resolutions of few tens of nm. A similar strategy60 hasbeen developed with 193 nm water immersion lithography whichallows photoresist features of 100 nm or even smaller to beeasily obtained, to be then used as templates for guiding theBCP assembly. Other materials for generating effective side-walls include metals and random copolymer brush layers asshown in Fig. 4a–d.61 Short chain PDMS brushes have been usedas well onto posts made of hydrogen silsesquioxane resist withEBL, which allows the self-assembly of PS-b-PDMS to be

controlled in two dimensional periodic patterns.62 In general,imposing lateral ordering and spatial connement in BCPs canbe easier than producing chemical pre-patterns with the aim oftemplating self-assembly and providing it with long-rangeperiodicity, and notwithstanding its simplicity this approachcan lead to an astonishing variety of possible functional struc-tures. The general merits of this class of methods include theremarkable thermal and mechanical stabilities of the templatestructures, and the possibility of using commercially availablecompounds for realizing the templates. Molds for NIL or solithography, with various shapes and geometries, can also be avaluable option for producing BCP-templating topographies.63

The stability of the pre-fabricated pattern against solvent andagainst temperature variations is of course especially importantto properly guide the self-assembly of the copolymers. In thisrespect, BCP self-organization can be even observed in moreexotic systems, for instance within the conned region of an

Fig. 3 Hybrid nanofabrication approaches involving BCPs. (a) Scheme showing the production of BCP multilayers. The red and blue domains indicate the PMMA andthe PS components in the BCP, respectively. (b–d) Corresponding micrographs of the resulting lamellar-forming PS-b-PMMA/cylinder multilayers, following the removalof the PMMA component by UV irradiation. Images are collected by scanning electron microscopy in a tilted (b) or top (c) view, respectively, or by transmission electronmicroscopy (d, top-view). (a–d) Adapted with permission from ref. 57, ACS Nano 2011, 5, 6164, DOI: 10.1021/nn2006943. Copyright ª 2011, American ChemicalSociety. (e–g) Soft-graphoepitaxy procedure, involving the preliminary, microscale patterning of a photoresist by conventional lithography (I-line), graphoepitaxy of aBCP film within the so-obtained photoresist microscale features (f), and pattern transfer into the underlying substrate (g). (e–g) Adapted with permission from ref. 59, J.Mater. Chem., 2011, 21, 5856, DOI: 10.1039/c0jm04248j. Copyright ª 2011, The Royal Society of Chemistry.




electrospun jet and of core–shell nanobers, resulting in theformation of internal patterns with well-dened phase-sepa-rated domains (Fig. 4e and f).64 More in general, the evaporativeassembly of properly conned organic solutions can be exploi-ted in a variety of hybrid lithographic methods which merge thetop-down control of capillarity and evaporation geometries andphenomena at solid–liquid and liquid–vapor interfaces with theself-assembly properties of the deposited, non-volatile solutes.The latter include many conjugated and active polymers, onwhich this results in the easy and low-cost achievement ofsolid patterns with sub-micron feature resolution by so-calledevaporative lithographies.65

However, also chemical or other hybrid strategies can lead toan intimate integration of different lithographic methods. Anexcellent example is schematized in Fig. 5a, which shows how torealize lithographically dened, chemically patterned polymerbrushes and mats or even features made of nanoparticlesimmobilized on the brushes.66 In this method, rst photoli-thography is performed on a photoresist deposited onto a cross-linked PS mat, and then etching is carried out by oxygen plasma(so-called breakthrough etch) and eventually by an additionalstep with a highly controllable lateral etch rate (so-called trimetch), for nely determining the feature size in polymers. This isthe top-down part of the overall fabrication sequence. Aer-wards, the remaining regions of the cross-linked PS mat withdimensions down to 10 nm leave exposed areas of the substrate

underneath, which allow endgraed polymer brushes [hydroxyl-terminated poly(2-vinylpyridine) or PS-PMMA randomcopolymer] to be graed from spin-coated lms, withoutaffecting the polymer mat chemistry. Finally, the resultingchemical pattern can be employed to direct site-specicimmobilization of metal nanoparticles or overlying PS-b-PMMAself-assembly, completing the ultimate, bottom-up part of themethod. The main merit of this method is the very high preci-sion in the denition of the chemical patterns at the 10 nmscale. Another recently proposed hybrid method exploitsinstead a smart combination of serial solvent annealing andEBL to induce the formation of coexisting sub-10 nm sphericaland cylindrical domains in a PS-b-PDMS lm, as schematized inFig. 5b.67 Here, the exposure of a pre-deposited copolymer layerto either acetone or dimethylformamide (DMF) vapors leads tothe formation of either cylindrical or spherical micro-domains,due to the differences in the swelling contrast of PS and PDMSin the two solvents. Interestingly, this assembly behavior isreversible, therefore successive exposures to different solventslead to repeatedly switching morphologies. Then EBL is used tocross-link and stabilize the ultimately obtained morphology byspatially selective irradiation. Finally, a second exposure tosolvent vapors can produce complementary patterns in the non-crosslinked regions as illustrated in Fig. 5b.

The transfer printing concept,68 schematized in Fig. 6, isinherently hybrid as well, and directly descends from original

Fig. 4 (a–d) Graphoepitaxy for directed self-assembly of BCPs by chemically heterogeneous (a) or homogeneous (b) topographies. (c) and (d) show differentmagnifications of PS-b-PMMA on a chemically homogeneous topographic pattern made of a random copolymer. In the schemes the red and blue domains indicate thePS and the PMMA component in the BCP, respectively. Adapted with permission from ref. 61, Adv. Mater., 2010, 22, 4325, DOI: 10.1002/adma.201001669. Copyrightª2010, WILEY-VCH Verlag GmbH & Co. KGaA. (e and f) BCP phase separationwithin electrospun nanofibers, axial views by axial transmission electronmicroscopy. Imagesshow the cross-section of fibers realized by coaxial electrospinning of poly(styrene-block-isoprene-block-styrene) and a poly(methyl methacrylate-co-methacrylic acid)random copolymer with higher Tg, and subsequent annealing (e). As an alternative, poly(styrene-block-isoprene) can be used for the core of fibers, producing well-defined spherical isoprene microdomains upon annealing (f). Scale bars ¼ 100 nm. Adapted with permission from ref. 64, Nano Lett., 2006, 6, 2969–2972, DOI:10.1021/nl062311z. Copyright ª 2006, American Chemical Society.




mCP methods. This approach can be implemented in an eitheradditive (as in mCP, Fig. 6a) or subtractive (Fig. 6b) way; it isextremely exible in terms of usable materials and compounds(including organic semiconductors), and has many differentvariants each addressing the needs of specic device platforms.

It is especially useful for device fabrication in exible elec-tronics and optoelectronics, and for realizing lithographicallydened features of sensors or chips implantable in the body. Inall cases, the contact of a stamp with a substrate leads to thedelivery of a material from a source surface to a target surface.In additive printing, the source is the elastomeric surface, andthe inkmaterial is delivered onto the substrate following contact.The roles of the stamp and the substrate are of course inverted insubtractive methods. It is clear that interfacial energies play apivotal role in determining the release of the ink material fromone surface to another. Deterministic assembly processes are alsopossible (Fig. 6c), in which the elastomeric mold is placed incontact rstly with a pre-patterned substrate from which itselectively retrieves some ink features, and then with a targetsubstrate onto which the nanostructures or microstructures arenally delivered. This last variant combines subtractive andadditive operations and has the merit of greatly reducing theoverall amount of used material during transferring.

A very beautiful, recent example of lithographic integrationmerging together transfer printing and BCP lithography isbased on using PDMS molds for directing the self-assembly ofcopolymers (Fig. 7a).69 Here, the process consists of realizingPDMS molds with 1 mm wide recessed regions from siliconmasters, depositing PS-b-PDMS by spin-coating onto thepatterned side of the elastomeric elements, transferring thewhole, 1 mm wide polymeric feature onto the substrate ofthe target substrate, and nally removing the elastomeric moldto transfer the self-assembled BCP pattern on the substrate(Fig. 7b). The eventual selective removal of one of the two

Fig. 5 (a) Schemes showing the formation of a chemical pattern, followed by nanoparticle immobilization or directed assembly. A film of cross-linked PS is patternedand etched, and a hydroxyl-terminated random copolymer of styrene and methyl methacrylate is grafted onto the exposed spaces of the substrate. Nanoparticleimmobilization and BCP directed assembly are also enabled in this way. Adapted with permission from ref. 66, Macromolecules, 2011, 44, 1876, DOI: 10.1021/ma102856t. Copyright ª 2011, American Chemical Society. (b) Schematics of the formation of dual morphologies in PS-b-PDMS thin films through solvent annealing,electron-beam irradiation, and a subsequent solvent annealing. Cylindrical and spherical morphologies are formed by exposing the film to vapors of acetone and DMF,respectively. Adapted with permission from ref. 67, Nano Lett., 2011, 11, 5079, DOI: 10.1021/nl203445h. Copyright ª 2011, American Chemical Society.

Fig. 6 Schemes showing different transfer printing modalities: additive (a),subtractive (b), and deterministic assembly (c). Reproduced with permission fromref. 68, Adv. Mater., 2012, 24, 5284, DOI: 10.1002/adma.201201386. Copyrightª2012, WILEY-VCH Verlag GmbH & Co. KGaA.




components as in standard BCP lithography leads to very high-resolution topographic features (sub-10 nm).

A different strategy is based on the photoplasticization ofazo-compounds, occurring by repeated photoisomerization ofazobenzene molecules, which can be used for patterningpurposes as in directional photouidization lithography.70 Thisapproach is directly built on the multifunctional properties ofexposed organics, which have to show photomechanical effectsleading to molecular alignment and mass transport and diffu-sion, following irradiation under given wavelength and polari-zation conditions. A nice example of the achievable pattern isprovided by the complex evolution of the surface topography ofpoly(disperse orange 3) azo-polymer features under light irra-diation, with various combinations of the polarization of twoincident beams (Fig. 8).70 An important feature of this approachfor controlling surface features is of being basically additive,not requiring chemical development or etching to removeundesired polymer regions. In particular, the initial featurescan be obtained by many assembly methods including solithographies and especially MIMIC, which is additive, as

shown in Fig. 8. Hence, the overall process minimizes the wasteof functional material, and is exible and cheap.

Indeed, as pointed out by Whitesides, microuidic technol-ogies used for lithographic purposes are inherently additive andgreatly reduce material waste. The achieved resolution is relatedto the used, elastomeric channels which in turn depends on thepreviously fabricated master structures.13 In this respect, newpossibilities have been opened to enhance resolution andfabricate functional polymer particles by integrating exposure-based curing methods with microuidics. This is at the base ofthe continuous ow lithography (CFL) proposed by the Doylegroup.71,72 In this technique, projection photolithography iscarried out on negative resists which ow in microuidicchannels. This leads to the rapid photo-polymerization of solidparticles having controlled shapes, which can be nally sepa-rated from the liquid resist, collected and used as self-standingfunctional micro-objects. Chemical anisotropy and multi-functionality can also be obtained in individual particles, per-forming photo-polymerization across sharp, heterosolutioninterfaces present in microchannels due to laminar ow(Fig. 9a–c).72 Recently, this approach has been combined withtwo-photon polymerization, which allows microfabricated poly-mer helices and other three-dimensional, complex activeparticles to be realized (Fig. 9d).73

A different method to obtain three-dimensional, hierarchi-cally ordered microparticles is based on the combination ofholographic lithography (HL) on a photosensitive resist andsurfactant templating as schematized in Fig. 10.74 Here, thepolymer is exposed in 0.15 s with a controlled distribution of anelectromagnetic eld, given by constructive and destructiveinterference of multiple coherent beams. Such an opticalpattern is in turn generated by means of an array of light-refracting microprisms produced in PDMS by replica moldingfrom a master with an inverted pyramid-shaped groove array.Then, the patterned polymer serves as a template for inltratinga silica precursor with a lyotropic surfactant phase such astetraethylorthosilicate and a triblock copolymer. The nalcalcination and polymer removal lead to free-oating particlesexhibiting a mesoporous structure which is the inverse replicaof the original holographic pattern. The main advantage of thisapproach is the simple generation of the hierarchical structureswith high degree of control.

Fig. 7 (a) Transfer printing of a BCP self-assembly pattern. Directed assembly is induced by a PDMSmold before additive transfer printing. (b) Photograph of a resultingBCP sample. Adapted with permission from ref. 69, Adv. Mater., 2012, 24, 3526, DOI: 10.1002/adma.201200356. Copyright ª 2012, WILEY-VCH Verlag GmbH & Co.KGaA.

Fig. 8 Fabrication of surface relief gratings (SRGs) by MIMIC followed by holo-graphic photofluidization lithography on azopolymers. Inset: scanning electronmicrograph. Reproduced with permission from ref. 70, Adv. Funct. Mater., 2011,21, 4412, DOI: 10.1002/adfm.201101203. Copyright ª 2011, WILEY-VCH VerlagGmbH & Co. KGaA.




The integration of electrospinning with exposure-basedlithographies such as optical lithography, two-photon, or EBL,allows researchers to develop interesting hybrid technologiesand to even obtain new organic–inorganic nanomaterials. Manydifferent dopants can be mixed in electrospun solutions inorder to obtain nanobers made of blends or composites.Possible dopants include low-molar mass molecules such asbiological compounds, uorescent compounds, laser dyes,dispersed carbon nanotubes, graphenes, or semiconductor,metallic or oxide nanocrystals. When nanoparticles are used,

however, their aggregation in solution, the possible clogging ofthe electrospinning die, and the eventual non-uniform distri-bution of the nanocrystals in the ultimately resulting nanoberscan constitute major problems preventing to fabricate reliablenanocomposite materials. Analogous issues, basically related tothe unfavourable rheological properties of uids with a signif-icant solid component provided by nanoparticles, are oenencountered in embossing methods and other lithographicapproaches based on material ow. A largely applied strategy toovercome unfavourable ow conditions consists of in situsynthesis of nanoparticles inside solid samples. In this way,solid nanoparticles are obtained in the matrix of polymernanobers aer electrospinning, or in polymeric patterns aermolding processes, and this allows researchers to perform thelithographic fabrication steps under low-viscosity conditions,which helps to increase resolution and disfavours the formationof defects. This approach passes through the synthesis of suit-able molecular precursors, which can be added to solutions orprepolymers without altering remarkably the pristine rheology,and which allows nanoparticles to be obtained followingdecomposition processes. A class of composite nanobersrealized in this way is made by PMMA and embedded CdSnanocrystals synthesized by in situ thermal treatments75 orEBL76 on cadmium thiolate precursors. Other examples includein situ approaches based on exposing electrospun bers to asuitable gas or solution, thus inducing the formation of nano-particles. In this way, for example, nanobers spun from poly-mer/lead acetate ethanol/water solutions and then exposed toH2S result in nanocomposites with PbS nanoparticles.77 Makingthe exposures spatially selective by suitable masking or tem-plating elements would lead to the integration of bottom-upand in situ nanocrystal growth with top-down patterningcontrol.

4 New directions in hybrid devicefabrication

The intrinsic difficulty of bottom-up routes is the lack of orderor patterning effectiveness at sufficiently long ranges, whichgenerally makes these techniques hardly applicable to theproduction of real devices. This has motivated the developmentof various templating and imprinting approaches to drive andenhance self-assembly phenomena as described above. Anothergenerally followed route, allowing realization of microelectronicdevices based on conjugatedmaterials, consists of coupling top-down deposited metal electrodes with active organic materials,whichmay include single crystals organized in two-dimensionalarrays due to pre-patterned, microcontact-printed domains ofoctadecyltriethoxysilane,78 light-emitting or conductive polymernanobers generated by electrospinning79–81 or by solvent-resistant so lithography, and vacuum-sublimated molecularlayers.82 In all these processes, bottom-up self-assembly ofconjugated molecules is either exploited directly or it can beinduced or even templated by proper so lithographic stepscarried out during device fabrication. Similar architectures ofelectrodes contacting individual83 or even dense arrays84 ofelectrospun piezoelectric nanobers made of PVDF or of its

Fig. 9 (a–c) Various multifunctional polymer particles realized by CFL withhydrodynamic focusing. Adapted with permission from ref. 72, Angew. Chem., Int.Ed., 2010, 49, 87, DOI: 10.1002/anie.200905229. Copyright ª 2010, WILEY-VCHVerlag GmbH & Co. KGaA. (d) Fluorescent polymer helices realized by two-photonCFL. Adapted with permission from ref. 73, Adv. Mater., 2012, 24, 1304, DOI:10.1002/adma.201103357. Copyright ª 2012, WILEY-VCH Verlag GmbH & Co.KGaA.

Fig. 10 Scheme of converging HL and surfactant templating for the realizationof (i) hierarchical ordered array and (ii) free-floating microparticles. (b) Structureevolution from the polymeric HL structure as featured by a microprism array, to aporous structure filled with a precursor, and finally to a hierarchically orderedmacro–mesoporous structure obtained upon polymer template removal. Repro-duced with permission from ref. 74, Chem. Mater., 2010, 22, 4117, DOI: 10.1021/cm1009527. Copyright ª 2010, American Chemical Society.




thermally stable copolymer, poly(vinylidene uoride–triuoro-ethylene), allow multifunctional devices to be obtained. Thesecan be used as nanogenerators for mechanical energy harvest-ing, as accelerometers or as ultra-sensitive strain or pressure/force sensors.

NIL can be applied to make easier the fabrication of polymerthin-lm transistors as shown in Fig. 11a–f.85 Here, the rst partof the process consists of depositing a buffer resist (poly-methylglutarimide) and a top PS layer, imprinting by a three-dimensional silicon mold, and carrying out sequential dry andwet etching steps to transfer part of the pattern to the otherlayers underneath. This leaves exposed metal regions, whichwill serve as source and drain electrodes, and lateral features ofinert polymers, which will serve as banks for the subsequentink-jetting of semiconductor materials. The second part of theprocess relies on surface treatments allowing a wettabilitycontrast to be obtained, and nally ink-jet printing86 of aconjugated polymer. The spin-coating of a dielectric layer andprinting of the gate electrode complete the transistor fabrica-tion that critically benets from the self-alignment of differentfeatures as provided by the initial three-dimensional mold(which has to exhibit signicantly protruding edges or featuresand allows one to avoid overlay registration steps in multilayerdevice production),87 and by a close integration of NIL, use ofsurface modiers, and ink-jet printing of active polymers.85

Other devices based on active polymers are based on multilayer

patterning through different and combined methods. A recentlyproposed demonstrator is a polymer distributed feedback laserwhose emission can be tuned by means of applied electric eldsand embedded layers made of non-linear optical (NLO) chro-mophores (Fig. 11g–i).88 These devices are produced by multi-layer patterning steps including room-temperature NIL andphotolithography.

Device applications of BCP lithographic methods areintriguing as well and include metal–oxide–semiconductorcapacitors exhibiting capacity signicantly increased (30%)compared to planar reference structures,89 eld effect transis-tors based on multiple nanowires,90 memories, and so on.Furthermore, free-standing masks for etching and porousmembranes can be realized by spin-coating PS-b-PDMS formingcylinders whose longitudinal axis is oriented perpendicularly tothe lm surface, detaching such a lm from the underlyingsubstrate by HF, and carrying out domain-selective treatmentswhich include the oxidation of the PDMS blocks and theremoval of the PS component by oxygen plasma.91 Otherimportant applications are given by organic solar cells, whereBCPs can be utilized as active materials given that suitablesemiconductor or conjugated components are incorporated inthe copolymers, and by hybrid organic–inorganic photovoltaicdevices where BCP lithography can be a powerful tool to realizeordered patterns and improved bulk heterojunctions.92 Similarconsiderations hold for organic light-emitting devices.

Fig. 11 (a–f) Scheme of the realization of a polymer field-effect transistor by self-aligned, one-step, multilayered patterning. Adapted with permission from ref. 85,Adv. Mater., 2011, 23, 4107, DOI: 10.1002/adma.201101291. Copyrightª 2011, WILEY-VCH Verlag GmbH & Co. KGaA. (g–i) Different cross-sectional view and schemeof an electrically tunable, organic distributed feedback laser device produced by multilayer patterning through photolithography and room temperature NIL. The scalebars are 500 nm (g) and 3 mm (h), respectively. Adapted with permission from ref. 88, Adv. Mater., 2012, 24, OP221, DOI: 10.1002/adma.201201453. Copyrightª 2012,WILEY-VCH Verlag GmbH & Co. KGaA.




In device fabrication, an oen critical parameter is theroughness of all the surfaces involved (metal, polymer, semi-conductor, etc.). Too high roughness values can determineundesired lithographic pattern transfer, disconnected metalpads, bad organic–inorganic contacts, light-scattering andincreased optical losses in microfabricated waveguides, and soon. In this respect, lithographic methods based on directed self-assembly of BCPs have been recently demonstrated to enable atight control of the ultimate surface roughness.93 This approachworks by means of positively charged polymersomes made ofpoly(2-(N,N-dimethylamino)ethyl methacrylate)-b-poly(tert-butylmethacrylate) (PDMAEMA-block-PtBMA) through reversibleaddition–fragmentation chain-transfer polymerization (Fig. 12a),solution self-assembly (Fig. 12b) and nally directed self-assembly on a target surface (Fig. 12c,d). While the initialassembly of the polymersomes on surfaces can even lead to anincrease of the surface roughness at the nanoscale, calibratedannealing steps allow one to achieve a signicantly reducedroughness, with root mean square values down to the scale of0.1 nm.

A different procedure sharing some of these ideas is at thebase of so-called self-perfection by liquefaction (SPEL, Fig. 12e).94

Here, polymer features undergo melting for a very short timeinterval (hundreds of ns), and the resulting decrease of viscosity(basically analogous to that of NIL in terms of the underlyingglass-transition physics) is exploited to reassemble the nano-structures and to obtain a signicantly reduced line-edgeroughness (down to the scale of 1 nm). The process can beperformed with one or more plates placed in contact or with agap above the polymer feature, thus guiding the reassembly stepas schematized in Fig. 12f.

Finally, various methods have been proposed to enhance thepatterning capacity on or by functional polymer nanobers,which may be of great utility for realizing a wide range of

photonic and electronic devices. Interestingly, differentapproaches exploit different physical properties of the nano-bers, whichmay open further ways for lithographic integrationin the near future. For instance, ash welding is an interestingprocessing method, in which conductive polymer nanobersmade of polyaniline and with quite low (bulk-like) thermalconductivity and simultaneously efficient photothermalconversion are rapidly exposed to ash light (with ms exposuretimes) and form welded regions and ultimately continuouslms.95 Irradiating through optical masks or grids can thus leadto nanober-mediated polymer patterning with obvious poten-tial device applications. A completely different method is basedon performing room temperature-NIL on conjugated polymer,light-emitting electrospun nanobers (Fig. 13).96,97 Demon-strated resolutions are of the order of 100 nm. Devices poten-tially realizable in this way include plasmonic resonators,organic nanober light-emitting devices with enhanced photonextraction efficiency or highly controlled emission direction-ality, polymer nanober distributed feedback lasers andphotonic crystals. In addition, nanopatterning leads to tailoringthe polarization of light emitted from the nanobers. A reduc-tion or an enhancement of the polarization ratio of emittedlight is found upon imprinting wavelength-scale periodic grat-ings with features perpendicular or parallel to the ber length,respectively.

5 Conclusions

In this feature article, we have limited our description to themain techniques which allow the hybrid character of the overalllithographic processes to be clearly appreciated. Many otherprocesses, however, are possible and would deserve to bementioned in the broader framework of multifunctionalpolymer nanostructures, including, for instance, self-assembly

Fig. 12 Roughness tailoring approaches. (a) Preparation of PtBMA and chain extension of a PtBMA macroinitiator. (b) Self-assembly of PDMAEMA-block-PtBMA intopolymersomes. (c and d) Modification of a flat (c) and a rough (d) negatively charged surface with positively charged polymersomes, and roughness reduction uponannealing. Adapted with permission from ref. 93, Adv. Funct. Mater., 2013, 23, 173, DOI: 10.1002/adfm.201200564. Copyrightª 2013, WILEY-VCH Verlag GmbH & Co.KGaA. (e and f) Schemes of open and capped SPEL. Adapted with permission from ref. 94, Nat. Nanotechnol., 2008, 3, 295, DOI: 10.1038/nnano.2008.95. Copyright ª2008, Macmillan Publishers Ltd.




technologies to fabricate active polymer nanobers and nano-wires such as those using polycarbonate or alumina porousmembranes98 and so templates.

In general, a signicant number of studies have alreadyyielded an interesting degree of integration between comple-mentary lithographic strategies on or through multifunctionalpolymers. It is clear, however, that a lot of challenges stillremain open in order to lead the resulting devices to a moreadvanced development stage and then to the market. Inparticular, the embedment of patterned active polymers, or low-dimensional, functional organic structures with device archi-tectures of actual practical use will need much more effort toimprove the reliability and reproducibility of the obtainedsystems and interfaces, and better established lithographicinterfaces to couple bottom-up organized macromolecules atthe nanoscale with top-down dened systems of electrodes,exible microscale components, addressable matrices and largeoptical circuits to be handled macroscopically. Materialsincreasing the reproducibility and accuracy of so lithogra-phies, such as elastomers with improved mechanical stability,can be an important strategy to further improve the reliability oflithographic results on active and multi-functional polymers.The increasing availability of specically designed equipmentand lithographic tools, enabling multi-level so lithographiesand NIL, will improve throughput and make easier the reali-zation of multi-layer devices based on functional organics.Electrospinning is still performed on a largely empirical basisfor many active compounds. For this and other processes, amore in-depth comparison of results from extensive experi-mental campaigns and ndings from modelling and simula-tions will allow researchers to improve their knowledge of themany operational variables, and nally the overall nano-fabrication reliability. Once optimized, electrospinning couldbe fruitfully combined with a complementary lithographicplatform, including BCP-based and transfer printing methods,to control the position of individual nanobers on devices, andto assemble complex photonic circuits. Indeed, applicationsmay be the driving force for bringing hybrid patterning

approaches on functional polymers closer to volume devicemanufacturing and to commercialization, particularly in theelds of exible electronics, optoelectronics, and nano-photonics. The outlook for the related classes of devices iscertainly good in the medium term, given the fact that activepolymers can bring a signicant added value to many of theseapplications. To be accomplished, these exciting objectives willcertainly require the strengthening of a truly transversal way ofthinking in terms of so matter processing, as may be guar-anteed only by genuinely cross-disciplinary teams ofresearchers, materials scientists and engineers.

Acknowledgements

We acknowledge the European Research Council for support-ing, under the European Union's Seventh Framework Pro-gramme (FP7/2007–2013), the ERC Starting Grant “NANO-JETS”(grant agreement no. 306357). The support from the ItalianMinistry of Education, Universities, and Research through theFIRB Projects RBNE08BNL7 ‘Merit’ and RBFR08DJZI ‘Futuro inRicerca’ is also acknowledged.

Notes and references

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