Patterned polymer brushes.

TL;DR: This critical review summarizes recent developments in the fabrication of patterned polymer brushes as top-down lithography reaches the length scale of a single macromolecule and the combination with the bottom-up synthesis of polymer brushes by surface-initiated polymerization becomes one main avenue to design new materials for nanotechnology.

Abstract: This critical review summarizes recent developments in the fabrication of patterned polymer brushes. As top-down lithography reaches the length scale of a single macromolecule, the combination with the bottom-up synthesis of polymer brushes by surface-initiated polymerization becomes one main avenue to design new materials for nanotechnology. Recent developments in surface-initiated polymerizations are highlighted along with diverse strategies to create patterned polymer brushes on all length scales based on irradiation (photo- and interference lithography, electron-beam lithography), mechanical contact (scanning probe lithography, soft lithography, nanoimprinting lithography) and on surface forces (capillary force lithography, colloidal lithography, Langmuir–Blodgett lithography) (116 references).

Summary

1. Introduction

• The fabrication of patterned polymer brushes on solids at the micro- and nanometre scales, with a controllable physicochemical property at a molecular level, has moved into the focus of materials science and engineering in micro- and nanotechnology.
• The ‘‘grafting-to’’ approach involves the experimentally simple process of end-functionalized polymer chains to react with an appropriate substrate.
• A wide variety of head groups allow the attachment of initiator functions for all known types of SIP and it comes in handy that powerful techniques are already developed to prepare patterned SAMs.
• The goal of this review is to introduce the reader with existing lithographic techniques and their combination with surface-initiated polymerization to create patterned polymer brushes as functional surfaces.

2. Photo and interference lithography

• Lithography using irradiation of UV light, X-rays, electrons as well as ions is a widely used technique for the fabrication of micro- and nanostructured materials.
• As a matured technique in industry, photolithography generally involves the transfer of a mask pattern onto a substrate over large areas coated with a light sensitive polymeric photo resist and subsequent selective chemical removal of the resist.
• The remaining patterned resist is then used for a selective etching or deposition process.
• The resolution for photo lithography is generally determined by the diffraction limit, which is a feature size of about half the wavelength of the light used.
• As a consequence, especially in microchip fabrication companies pushing the limits of UV photolithography with UV sources of decreasing wavelength to fulfill Moore’s law with established technology.

2.1 UV lithography

• Rühe et al.36 first realized the potential possibility in using photo (UV) lithography to fabricate patterned polymer brush microstructures by photo SIP in a bottom-up approach.
• They used a SAM of azo-functionalized alkylsilanes of AIBN-type as the photosensitive layer and irradiated the SAM through a mask.
• The patterned diamond surface with oxidized and native areas allowed selective SIP of styrene and other vinyl monomers by means of SIPGP.
• After deactivation of the polymer chain ends byNaN3, and removal of brush regions by UV photodegradation through a mask, the native substrate areas were backfilled with the initiator SAM for a second SI-ATRP to result in a patterned binary brush covering the entire surface.

2.2 Interference lithography

• Interference lithography (IL) is a mask-free technique for patterning regular arrays of fine feature resolution for a certain wavelength without the use of complex and expensive high numerical aperture optical systems.
• This technique has the advantage of practically unlimited depth of focus and very large exposure fields.
• Generally, a linear fringe pattern with a sinusoidal intensity distribution could be formed with two or more coherent beams.
• 40,41 This strategy was firstly exploited by Padeste et al.40 who used EUV light in a synchrotron-based interference setup to create the initiator radicals in periodic line space or dot arrays.
• 42,43 Although a number of methods have been exploited to create gradient assemblies using short organic modifiers, relatively few techniques are available for generating gradient polymer brushes that rely on selective physical or chemical treatment of surfaces before or during growth of a polymer brush.

3. Electron-beam lithography (EBL)

• To realize further performance enhancement of integrated circuits one central strategy in the microelectronics industry is still to fabricate structures with smaller dimensions to cope with Moore’s law.
• This is a driving motor for the development of lithographic technology using irradiation of decreasing wavelength.
• 47 Except for maybe scanning probe lithography, the resolution of EBL has not been surpassed by any other Fig. 3 (A) Outline of patterned/gradient polymer brushes by UV-interference lithography using a UV laser for IL.
• Such as high cost of the instrumentation, the need of ultra high vacuum for operation, and the inherently serial patterning, EBL is the only technique to create patterns of microscale periodicity with nanometre precision.

3.1 Electron beam resist lithography

• EBL is almost exclusively used in resist approaches and identical as those for conventional photo resist using PMMA as the resist.
• The fabrication of patterned polymer brushes at the nanoscale using EBL with resists and pattern amplification by SIP was firstly reported by Zauscher et al.18.
• Patterns with controlled feature size, shape, and periodicity could be created even over larger areas.
• Moreover, the surface chemistry contrast of gold patterned silicon substrates facilitates the fabrication of binary polymer brushes with high lateral resolution by using surface selective silane and thiol-based initiator SAMs.
• This method is not limited to PMA polymer brush systems but applicable to polymers that show positive tone behavior under e-beam exposure.

3.2 Electron-beam chemical lithography (EBCL)

• While in ‘‘lift-off’’ EBL, the surface materials contrast was used to realize patterned brushes, electron beam chemical lithography (EBCL) as developed by Eck et al.53 allows the introduction of the chemical contrast within the SAM itself and thus avoids overlaying topographical features.
• 2.2 Patterned SAMs for initiator free SIP: self-initiated photografting and photopolymerization .
• Only at longer photopolymerization times, branching and crosslinking of the grafts become apparent27,56 and surface grafting stagnates.

3.3 Electron beam induced carbon deposition

• There are several drawbacks associated to the SAM system such as their limited thermal and chemical stability.
• Since, in SIPGP, the monomer itself acts as a photosensitizer leading to surface radicals by abstraction, virtually any organic layer that is locally deposited can act as a 2D template for regio-selective grafting.
• Taking advantage of the high resolution of e-beam writing and the fact that electron irradiation of any surface (except gold) leads to deposition of carbon with a composition close to C9H2O with 90% sp 2 and 10% sp3 carbon.

4. Scanning probe lithography (SPL)

• Scanning probe lithography (SPL) uses a scanning tunneling microscope (STM) or the atomic force microscope (AFM) to fabricate nanometre-scale features.
• Moreover, SPL can be carried out in ambient conditions of temperature and pressure and can be performed in different solvent or buffer environments with a minimum of sample preparation.
• The majority of patterning efforts using SPL has been directed towards fabricating templates for subsequent modification with initiators Fig. 7 (A) Scheme of carbon templating (CT) to create patterned brushes of controlled morphology directly on native substrates.
• (B) AFM analysis of a gradient carbon deposit, and the resulting gradient PS brushes.

4.1 Dip-pen nanolithography

• The molecules on the AFM tip are transported to the substrate by a diffusional mechanism.
• DPN can generate thiol SAM patterns in a dry nitrogen environment, since a water meniscus is always present, even at 0% relative humidity.
• In the case of hydrophilic molecules, such as 16-mercaptohexadecanoic acid (MHA), it allows thiol transport to the gold surface.
• 82 Liu et al.83 combined DPN and ROMP to fabricate polymer brush arrays on the nanometre length scale with great control over feature size, periodicity and shape (Fig. 8).
• This approach yielded conductive pEDOT nanostructures with sub-100 nm dimensions on semiconducting and insulating surfaces.

4.2 Nanoshaving and nanografting

• While imaging with the AFM in contact mode, the force between the tip and the sample is a major concern as it may cause significant damage to the sample.
• The large contact pressure during patterning causes the displacement of molecules.
• Liu et al.79 determined important parameters for nanoshaving and nanografting such as the scanning force and speed, the concentration of alkanethiol solution, and the sharpness of the AFM tip.
• The resulting patterns can be used as templates for SIP.
• Analog to the CT approach29 and the 3D-morphology control of nanopatterned polymer brushes by EBL,20,21 Zheng et al.87 employed DPN for the regio-selective deposition of ATRP initiators.

4.3 Anodization lithography

• AFM anodization lithography is an electrochemical lithography process in which a voltage bias applied to an AFM tip, establishes a strong, localized electric field between the tip and substrate surface, and causes oxide growth on semiconducting silicon oxide substrates.
• The factors affecting patterning are the applied electric potential between tip and surface, the relative humidity, the electronic state of tip and surface materials, and the patterning speed.
• (B) AFM height images and corresponding typical height profiles of a PNIPAAM brush line nanopattern22 (reproduced with permission from ref. 22, copyright 2004, American Chemical Society).
• Fig. 10 (A) Stepwise fabrication schemes of nanopatterned pENB and pCOT by using anodization lithography and ROMP.
• New anodic oxide patterns, next to already existing polycot nanopatterns, can be generated and then amplified the new pattern by ROMP of ENB, using Grubbs’ catalyst.

5. Soft lithography

• Soft lithography represents a non-photolithographic strategy based on self-assembly and replica molding for micro- and nanofabrication.
• It provides a convenient, effective, and low-cost method for the formation and manufacturing of micro- and nanostructures even over large areas, and has since been used by countless research groups.
• Crucial for mCP is the conformal contact of the stamp with the substrate surface.
• They initially printed a non-reactive SAM of CH3–(CH2)15SH onto a gold surface with a selective backfill of a second thiol of HO(CH2CH2O)2(CH2)11SH onto the bare gold regions.
• An interesting approach to fabricate patterned multi-component polymer brushes of high complexity was demonstrated by Huck et al. (Fig. 11).25 First, a patterned initiator SAM was prepared by mCP and used for SI-ATRP.

5.2 Extended micro contact printing

• To date, the development of mCP has exceeded the original aim of replicating PDMS stamp patterns.
• They are not accessible to a number of researchers because of expensive and complex instruments used.
• Huck et al.96 prepared hierarchically well-defined structured polymer brush microstructures via multiple step mCP with inks containing different ratios of inert along with initiatorfunctionalized thiols.
• Even more complex structures can be prepared by moving the stamp during the mCP printing process.
• After the back-filling with nonreactive thiol, a biological polynucleotide brush was grown by SIEP by incubating the patterned oligonucleotide SAM substrate into a mononucleotide solution.

6.1 Nanoimprint lithography (NIL)

• Nanoimprint lithography (NIL), initially invented and developed by Chou et al.102 in 1990s’, is a major breakthrough in nanopatterning because it has the advantage over other currently conventional lithography in producing sub-10 nm feature size over a large area with a high throughput and low cost.
• This is the key issue why NIL has attracted wide attention within only a few years after its inception.
• This fabricating strategy was then carried out to pattern polymer brush nanostructures by Carter and Hawker et al.,103 who employed a top-down nanocontact molding process, followed by the controlled growth of polymer brushes from these patterned features (Fig. 12).
• The primary patterning technique is a contact-molding process which involves the use of a patterned polymeric mold to template a secondary liquid photopolymer resin layer that is subsequently UV-polymerized while in contact with the mold to give pattern transfer.

6.2 Capillary force lithography (CFL)

• Capillary force lithography (CFL) is a simple and robust method that combines aspects of NIL and mCP.104 CFL, like mCP, uses an elastomeric stamp to transfer a pattern with high fidelity and in large scale onto a polymeric thin film but without the need of pressures, typical for imprint lithography.
• Once the polymer film is heated above its glass transition temperature, capillary forces cause the softened polymer to fill the open spaces of the elastomeric mold.
• Luzinov et al.105 reported the synthesis of binary polymer brush nanopatterns on a large scale by combining CFL with SIP.
• This technique was then developed by Luzinov and Zdyrko et al.106 to combine with solvent-assisted grafting approach to attach a poly-2-vinylpyridine (P2VP) onto a reactive surface forming a polymer brush by the ‘‘grafting-onto’’ method (Fig. 13).
• The patterned surfaces were obtained by protecting part of the reactive surface of epoxy functionalities poly(glycidyl methacrylate) (PGMA), followed by polymer grafting to the unprotected part of the surface.

6.3 Colloidal lithography (CL)

• It is well known that monodisperse colloidal microspheres easily self-assemble into hexagonal close packed arrays on surfaces as a result of capillary forces arising from the evaporation of solvents.
• By combination of colloidal self-assembly with nanofabrication techniques, 2D colloidal crystals have been employed as masks or templates for evaporation, deposition, and etching.
• The powerful lithographic tool has thus shown us a possibility to structure polymer brushes.
• Another combination of SIP and colloidal particles is to modify the particles to direct self-assembly of the particles into colloidal crystals and then use the stimulus-sensitive polymer brush to control the interparticle volume.

6.4 Langmuir–Blodgett lithography

• A Langmuir–Blodgett (LB) film contains one or more monolayers of an organic material at the air–water interface and can be directly deposited onto a solid by LB transfer.
• Depending on the conditions, a monolayer is adsorbed homogeneously with each immersion or emersion step.
• Control of the dynamic parameters during the transfer results in a direct variation of the pattern features.
• Depending on the initiator concentration, the periodicity and stripe width can be adjusted.

7. Conclusions and remarks

• The marriage of the top-down lithographic techniques with the bottom-up strategies of self-assembly and surface-initiated polymerization led to fantastic 2D and 3D structures of polymer brushes.
• The Royal Society of Chemistry 2012 many applications require side-by-side patterning on the microas well as the nanometre length scale and a defined periodicity.
• The challenge will be the reproducibility and degree of control.
• As outlined, most of the approaches to fabricate patterned polymer brushes rely on patterned SAMs as they provide a defined chemical handle for consecutive SIP on many surfaces.
• This additional step of SAM deposition along with the stability issues of many SAM systems limits the use of polymer brushes for technological and biomedical applications.

Figures

Fig. 15 (A) Formation of patterned polymer brushes by LB lithography. (B) AFM images of a sample before (B) and after (C) SIP of styrene114 (reproduced with permission from ref. 114, copyright 2007, Wiley-VCH Verlag GmbH & Co. KGaA).

Fig. 2 (A) Scheme of preparing structured PS brushes on UNCD. (B) AFM image of the resulting PS brushes33 (reproduced with permission from ref. 33, copyright 2007, American Chemical Society).

Fig. 13 (A) Scheme of patterned polymer brushes via CFL and solvent assisted grafting. (B) PS CFL on the PGMA surface (C) P2VP stripes obtained via solvent-assisted grafting106 (reproduced with permission from ref. 106, copyright 2008, Royal Society of Chemistry).

Fig. 8 (A) Preparation of poly(norbornene) brush by DPN and ROMP. (B) AFM image of polymer brush lines and dot arrays83 (reproduced with permission from ref. 83, copyright 2003, Wiley-VCH Verlag GmbH & Co. KGaA).

Fig. 4 (A) Stepwise fabrication of patterned PNIPAAM brushes created by EBL and SI-ATRP. (B–C) AFM scans of line patterns of gold, fabricated by EBL and subsequent PNIPAAM brush grown by SI-ATRP from immobilized thiol initiator on the Au18 (reproduced with permission from ref. 18, copyright 2004, Wiley-VCH Verlag GmbH & Co. KGaA).

Full text

3280 Chem. Soc. Rev., 2012, 41, 3280–3296 This journal is
c
The Royal Society of Chemistry 2012
Cite this:
Chem. Soc. Rev
., 2012, 41, 3280–3296
Patterned polymer brushes
Tao Chen, Ihsan Amin and Rainer Jordan*
Received 23rd August 2011
DOI: 10.1039/c2cs15225h
This critical review summarizes recent developments in the fabrication of patterned polymer
brushes. As top-down lithography reaches the length scale of a single macromolecule, the
combination with the bottom-up synthesis of polymer brushes by surface-initiated
polymerization becomes one main avenue to design new materials for nanotechnology.
Recent developments in surface-initiated polymerizations are highlighted along with diverse
strategies to create patterned polymer brushes on all length scales based on irradiation
(photo- and interference lithography, electron-beam lithography), mechanical contact
(scanning probe lithography, soft lithography, nanoimprinting lithography) and on
surface forces (capillary force lithography, colloidal lithography, Langmuir–Blodgett lithography)
(116 references).
1. Introduction
The fabrication of patterned polymer brushes on solids at the
micro- and nanometre scales, with a controllable physico-
chemical property at a molecular level, has moved into the
focus of materials science and engineering in micro- and
nanotechnology.
1
Because of low chain e ntanglement terminally
attached polymer brushes are the first choice for stimulus responsive
polymer coatings for sensor and actuator developments as
they react immediately to environmental changes, such as
solvent quality, pH, ionic strength, or temperature, with
significant changes of the polymer layer coating.
2,3
Polymer
brushes are ensembles of end-tethered polymer chains with
high grafting densities with respect to their radius of gyration
in which the high surface crowding results in considerable
stretching of the grafted chains from the substrate surface.
4
They are anchored to the substrate surface by either strong
physical absorption or covalent chemical attachment.
4
The
latter is preferred as it overcomes some of the disadvantages of
physisorption, such as solvent or thermal instabilities, and
Professur fu
¨
r Makromolekulare Chemie, Department Chemie,
Technische Universita
¨
t Dresden, Zellescher Weg 19, 01069 Dresden,
Germany. E-mail: Rainer.Jordan@tu-dresden.de;
Web: http://tu-dresden.de/chemie/mc; Fax: +49-351-46337122;
Tel: +49-351-46337676
Tao Chen
Tao Chen received his PhD in
polymer chemistry and physics
from Zhejiang University
(Prof. Li Wang group) in
2006. After his postdoctoral
training in Department of
Chemistry at University of
Warwick (Prof. Stefan A.F.
Bon group), he joined Prof.
Stefan Zauscher’s group at
Duke University as a research
scientist. He is currently an
Alexander von Humboldt
Research Fellow hosted by
Prof. Rainer Jordan at
Technische Universita
¨
t Dresden,
Germany. He is interested in the creation and manipulation of
stimulus responsive patterned bio-inspired polymeric materials and
self-assembled materials system for actuation and sensing
applications.
Ihsan Amin
Ihsan Amin is a postdoctoral
researcher in the group of Prof.
Rainer Jordan, Professur fu
¨
r
Makromolekulare Chemie,
Technische Universita
¨
t Dresden,
Germany. He finished his study
in physics at Rijkuniversiteit
Groningen, The Netherlands,
and completed his doctoral
studies at the Universita
¨
t
Bielefeld, Germany, in 2010
in the group of Prof. Armin
Go
¨
lzha
¨
user. His research
interests focus on the fabrication
and application of micro- and
nanopatterned stimuli responsive
polymer brushes with the main interest in the development of
‘‘Polymer Carpets’’, freestanding polymer brushes grown by
surface-initiated on crosslinked two-dimensional framework
nanosheets.
Chem Soc Rev
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The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 3280–3296 3281
offers greater control over grafting density and yields higher
packing densities.
Two fundamentally different approaches are used to realize
surface-attached polymer brushes. The ‘‘grafting-to’’ approach
involves the experimentally simple process of end-functionalized
polymer chains to react with an appropriate substrate. This
technique, however, has one intrinsic problem that often leads to
low grafting density and film thickness due to surface-screening
by already attached polymer chains. The ‘‘grafting-from’’
approach overcomes the shortcomings of the former as small,
low molar mass monomers are directly polymerized to form
the brush. Hence, surface-initiated polymerization (SIP)
5–8
yields polymer brushes of very high grafting densities that
render effectively the entire surface. Polymer brushes can be
used in surface-based technologies, such as switchable sensors
and actuators in micro- and nanotechnology, substrate for cell-
growth control, and for protein-resistant coatings in biological or
medical fields, etc.
2,3,8
Up-to-date most types of polymerization
reactions from free radical polymerization to highly defined living
polymerizations have been adopted to prepare polymer brushes
by SIP.
4–7
Because of the minute total amount of surface-bound
initiation sites, living ionic polymerizations from planar substrates
are experimentally very difficult to perform as they require ultra-
clean conditions even if a parallel sacrificial polymerization is
carried out in solution.
9,10
To realize structurally defined polymer
brushes comprising of linear chains of low dispersity, defined end
groups, block copolymer brushes by sequential monomer addition,
etc., surface-initiated controlled radical polymerization is the
method of choice as they are tolerant towards impurities and at
the same time offer a sufficient control of the polymer architecture
and composition. Hence, controlled radical surface-initiated
polymerization such as atom transfer radical polymerization
(SI-ATRP), nitroxide-mediated polymerization (SI-NMP) and
reversible addition–fragmentation chain transfer polymerization
(SI-RAFT) have become the most popular routes.
4–7
In this
context, a recent paper by Turgman-Cohen and Genzer
11
is
addressing the common practice in SI-ATRP to directly
correlate the molar mass and molar mass distribution of
grafted polymer to the polymer grown simultaneously in
solution/bulk. In contrast to the general assumption of an
equal polymerization behavior, their results from Monte Carlo
simulations indicate that bulk polymers grow at faster rates
and possess narrower molecular weight distribution than
polymers initiated from flat surfaces.
The formation of patterned polymer brushes is basically
straightforward: In a top-down approach a homogeneous polymer
brush is destructively patterned by selective lithography using
irradiation through a mask or simply by locally confined
mechanical force. However, lateral resolution will be quite
limited and debris of removed material is an issue. More
elegantly, a pre-patterned surface-bound initiator template
can be used to amplify a two-dimensional (2D) pattern into
a three-dimensional (3D) brush structure by SIP. With the
requirements for such a 2D template system that should be of
defined composition and end-function, irreversibly bound to
the substrate, ultrathin to allow patterning at any length scale
and of high reproducibility on a broad variety of substrate
types, self-assembled monolayers (SAMs) soon became the
dominant initiator system for SIP.
SAMs can be easily and reproducibly formed on almost any
substrate type giving the correct choice of the anchor group
and mesogen. A wide variety of head groups allow the
attachment of initiator functions for all known types of SIP
and it comes in handy that powerful techniques are already
developed to prepare patterned SAMs.
12
The two most popular
and best characterized SAM types are based on silanes to modify
hydroxylated surfaces s uch as glass or oxides and organosulfur
compounds, i.e. thiols to modify coin metals.
13–16
Depending on
feature size and substrate material used, patterned SAMs as
initiator templates for SIP can be prepared by a range of fabrication
strategies including photo and interference lithography,
17
electron-beam lithography (EBL),
18
electron-beam chemical
lithography (EBCL),
19–21
scanning probe lithography (SPL),
22,23
soft lithography,
24,25
etc. More recently, it was demonstrated that
patterned polymer brushes of defined 3D morphology can be
prepared even without a surface-bound initiator by self-initiated
photografting and photopolymerization (SIPGP)
26
on patterned
SAMs,
27,28
on carbon deposits (carbon templating, CT),
29–32
or
by the direct use of a surface chemical contrast.
31,33,34
The
scheme in Fig. 1 summarizes the various strategies for the
preparation of patterned polymer brushes.
The goal of this review is to introduce the reader with
existing lithographic techniques and their combination with
surface-initiated polymerization to create patterned polymer
brushes as functional surfaces. First, patterning of surfaces with
irradiation ranging from UV-light to electrons is presented.
Second, lithography techniques based on mechanical contact
such as soft lithography, scanning probe lithography and
nanoimprinting lithography are discussed and, finally, structure
formation based on surface forces such as capillary force
lithography, colloidal lithography and Langmuir–Blodgett
lithography is summarized.
Rainer Jordan
Rainer Jordan studied chemistry
at the University of Mainz
(Germany) and as an IAS-
fellow at Kyoto University
(Japan) with Prof. T. Saegusa.
He joined the research group of
Prof. K.K. Unger in Mainz and
worked as a PROCOPE-fellow
at the C.N.R.S. in Paris with
Prof. B. Sebille. In 1996 he
obtained his doctoral degree in
Chemistry with Prof. K.K.
Unger (Univ. Mainz). After a
postdoctoral stay with Prof.
Ulman at Polytechnic Univer-
sity in Brooklyn, NY (USA)
he was appointed as assistant professor. He returned to Germany
and joined the group of Prof. Nuyken at the Technische Universita
¨
t
Mu
¨
nchen for habilitation. Since 2008/2009 he is full professor at the
Professur fu
¨
r Makromolekulare Chemie at the Technische
Universit a
¨
t Dresd en, Germany. His res earch interests inclu de poly -
mer chemistry (tailored polymers), surface chemistry (self-assembled
monolayers and polymer brushes), biomimetic systems (artificial cell
membranes) , nanoscience (composites, colloidal systems) and nano-
medicine (polymer therapeutics and biomaterials).
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2. Photo and interference lithography
Lithography using irradiation of UV light, X-rays, electrons as
well as ions is a widely used technique for the fabrication of
micro- and nanostructured materials. As a matured technique
in industry, photolithography generally involves the transfer
of a mask pattern onto a substrate over large areas coated with
a light sensitive polymeric photo resist and subsequent selective
chemical removal of the resist.
35
The remaining patterned resist
is then used for a selective etching or deposition process. The
resolution for photo lithography is generally determined by
the diffraction limit, which is a feature size of about half the
wavelength of the light used. As a consequence, especially in
microchip fabrication companies pushing the limits of UV
photolithography with UV sources of decreasing wavelength
to fulfill Moore’s law with established technology.
2.1 UV lithography
Ru
¨
he et al.
36
first realized the potential possibility in using
photo (UV) lithography to fabricate patterned polymer brush
microstructures by photo SIP in a bottom-up approach. They
used a SAM of azo-functionalized alkylsilanes of AIBN-type
as the photosensitive layer and irradiated the SAM through
a mask. Free radical SIP (FR-SIP) occurred only at the
irradiated areas. As AIBN has a quite low extinction coefficient,
UV-induced decomposition of the initiator resulted in relatively
thin polymer brushes. Later, the same group introduced a more
suitable asymmetric azo-functionalized SAM featuring a methyl-
malonodinitrile and an aryl function with higher adsorption.
37,38
The thickness increase of the brush as a function of irradiation
time was found to be linear with final thickness values of up to
B400 nm after 24 h continuous UV irradiation.
Standard photolithography with an UV photo resist was
used by Jordan and Garrido et al.
33
to create a chemical
contrast on hydrogen-terminated diamond by plasma oxidation
(Fig. 2). The patterned diamond surface with oxidized and
native areas allowed selective SIP of styrene and other vinyl
monomers by means of SIPGP. As SIPGP is a self-initiated
polymerization and grafting reaction, it does not require a
surface-bond initiator but surface groups that can be easily
abstracted by a radical mechanism involving the monomer that
also acts as the photo sensitizer. The high difference of the bond
dissociation energies of groups in the oxidized and native
diamond surface areas resulted in highly selective formation
of poly(styrene) (PS) brushes only on the oxidized diamond. As
no intermediate SAM is needed, the PS brushes could be
subsequently converted under quite drastic reaction conditions
without noticeable detachment of the polymer. Thus, various
Fig. 2 (A) Scheme of preparing structured PS brushes on UNCD. (B) AFM image of the resulting PS brushes
33
(reproduced with permission
from ref. 33, copyright 2007, American Chemical Society).
Fig. 1 Overview of various strategies for the preparation of patterned polymer brushes (Abbreviations: SIPGP: self-initiated photografting and
photopolymerization; SIP: surface-initiated polymerization; CT: carbon templating; PL: photolithography; SA: self-assembly; EBCL: electron
beam chemical lithography; SPL: scanning-probe lithography; SL: soft lithography; NIL: nanoimprinting lithography; CFL: capillary force
lithography; CL: colloidal lithography; IL: interference lithography; EBL: electron beam lithography).
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The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 3280–3296 3283
functionalities, such as nitro, sulfonic, and aminomethyl groups
could be successfully incorporated at high yields.
With UV lithography and repetitive SIP, Liu et al.
39
created
a binary polymer brush pattern of poly(hydroxyethyl metha-
crylate) PHEMA and poly(methyl methacrylate) PMMA (or
poly(dimethylaminoethyl methacrylate) PDMAEMA) through a
two-step SI-ATRP. First, a homogeneous polymer brush was
prepared by SI-ATRP using an initiator SAM. After deactivation
of the polymer chain ends by NaN
3
, and removal of brush regions
by UV photodegradation through a mask, the native substrate
areas were backfilled with the initiator SAM for a second
SI-ATRP to result in a patterned binary brush covering the
entire surface. The principle of a chemical patterning was
further developed by Hawker et al.
17
They created discrete
areas of hydrophilic poly(acrylic acid) (PAA), and hydrophobic
poly(tert-butyl acrylate) (PtBA) brushes derived from grafted
PtBA homobrushes by photo lithography. In their approach, a
solution of PS containing bis(tert-butylphenyl)iodonium triflate
was spin-coated on top of a PtBA brush. UV irradiation
through a mask resulted in photo acid generation confined to
the irradiated areas. Diffusion of the photo acid caused local
deprotection of the tert-butyl ester groups within the brush and
resulted in a pattern of PtBA/PAA brushes.
2.2 Interference lithography
Interference lithography (IL) is a mask-free technique for
patterning regular arrays of fine feature resolution for a
certain wavelength without the use of complex and expensive
high numerical aperture optical systems. This technique has
the advantage of practically unlimited depth of focus and very
large exposure fields. Generally, a linear fringe pattern with a
sinusoidal intensity distribution could be formed with two or
more coherent beams. In an effort to overcome some drawbacks
of EBL including a limited choice of support materials that
allow the formation of SAMs, and to increase patterning
resolution for photo lithography, IL has thus been used to
combine with other radiation source lithography, such as UV or
extreme UV (EUV), to create patterned polymer brushes with
nanometre resolution over large areas.
40,41
This strategy was
firstly exploited by Padeste et al.
40
who used EUV light in a
synchrotron-based interference setup to create the initiator
radicals in periodic line space or dot arrays. The radicals are
created in a limited depth range of about a dozens of micro-
metre near the surface because of the high absorption of EUV
light by the substrate. In the subsequent polymerization reaction,
brushes were only grafted to exposed areas.
Gradient brushes with gradual variation of e.g. the graft
density, the molecular weight or the chemical composition
allow a systematic variation of surface properties across the
substrate and can help to improve the understanding of
topography- and/or chemistry-related phen omena.
42,43
Although
a number of methods have been exploited to create gradient
assemblies using short organic modifiers, relatively few techniques
are available for generating gradient polymer brushes that rely on
selective physical or chemical treatment of surfaces before or
during growth of a polymer brush.
42,44,45
This inclu des cre ation
of density gradients for growing polymers, gradual immersion
or withdrawal of a substrate from a polymerization solution,
regulation of the radiation intensity during UV exposure by a
shutter, etc.
42
Ru
¨
he et al.
41
recently presented an elegant
approach to generate gradient PMMA brushes with steep slopes
at length scales down to 100 nm combining UV-interference
lithography with SIP (Fig. 3). UV-interference is used to partially
deactivate a photo initiator SAM to obtain a gradient initiator
pattern. The remaining initiator is then used for surface-initiated
photo polymerization and resulted in gradient polymer brushes.
3. Electron-beam lithography (EBL)
To realize further performance enhancement of integrated
circuits one central strategy in the microelectronics industry
is still to fabricate structures with smaller dimensions to cope
with Moore’s law. This is a driving motor for the development
of lithographic technology using irradiation of decreasing
wavelength. From UV, the industry moved to deep-UV and
currently to EUV along with associated technological develop-
ments. EBL is currently discussed to realize further miniaturization,
however, EBL involves the development of new fabrication
equipment and change of process work-flow. Furthermore, it
might be to slow for chip mass production unless highly
parallel fabrication technologies can be developed. EBL
46
was already developed in the 1960s using existing scanning
electron microscope (SEM) technology and is now widely used
in research and special applications. EBL can be performed
using a mask or by direct writing with a focused electron-beam
for substrate patterning ranging from micrometres down to a few
nanometres.
47
Except for maybe scanning probe lithography,
the resolution of EBL has not been surpassed by any other
Fig. 3 (A) Outline of patterned/gradient polymer brushes by UV-interference lithography using a UV laser for IL. (B) 3D AFM height image of a
PMMA brush with crossed gradient structures
41
(reproduced with permission from ref. 41, copyright 2009, Wiley-VCH Verlag GmbH & Co.
KGaA).
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3284 Chem. Soc. Rev., 2012, 41, 3280–3296 This journal is
c
The Royal Society of Chemistry 2012
lithographic methods.
48
Although EBL has some drawbacks,
such as high cost of the instrumentation, the need of ultra high
vacuum for operation, and the inherently serial patterning,
EBL is the only technique to create patterns of microscale
periodicity with nanometre precision.
3.1 Electron beam resist lithography
EBL is almost exclusively used in resist approaches and
identical as those for conventional photo resist using PMMA
as the resist. The area irradiated by a focused electron beam is
chemically developed to reveal the underlying substrate for
selective etching and/or further modifications. The fabrication
of patterned polymer brushes at the nanoscale using EBL with
resists and pattern amplification by SIP was firstly reported by
Zauscher et al.
18
In their approach, a silicon surface is
patterned with gold using ‘‘lift-off’’ EBL (‘‘top-down’’) and
the resulting pattern is then amplified by SI-ATRP (‘‘bottom-up’’)
to obtain poly(N-isopropyl acrylamide) (PNIPAAM) from
immobilized thiol initiator SAM (Fig. 4). Patterns with controlled
feature size, shape, and periodicity could be created even over
larger areas. Moreover, the surface chemistry contrast of gold
patterned silicon substrates facilitates the fabrication of binary
polymer brushes with high lateral resolution by using surface
selective silane and thiol-based initiator SAMs.
In a similar approach Jonas et al.
49
prepared nanopatterned
surfaces by EBL and silane monolayers which were later used
for regio-selective growth of polymer brushes by means of
SI-ATRP.
50
The resulting height and width of the brush
nanopatterns are analyzed by the interplay of wetting and
stretching of the grafted chains at the pattern edges. Using
PMMA as the photo resist material for EBL, Ober et al.
51
recently
reported a direct patterning strategy of a series of methacrylate
polymer brushes with ester functions (poly(isobutyl methacrylate)
(PIBMA), poly(neopentyl methacrylate) (PNPMA), and
poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA)). It is well
established that patterning of positive tone methacrylate
photo resists by e-beam exposure is based on the scission
reactions that occur on the backbone chain.
52
The reaction
leads to the degradation of the polymer brush resist into
smaller fragments via a radical decomposition. The increasing
order of polymer sensitivity toward EBL was found to
form stable radicals upon irradiation (PMMA o PIBMA o
PHEMA E PNPMA o PTFEMA). By destructive EBL of
PMA brushes highly resolved nanostructured polymer brush
patterns down to 50 nm lines were obtained. This method is
not limited to PMA polymer brush systems but applicable to
polymers that show positive tone behavior under e-beam
exposure.
3.2 Electron-beam chemical lithography (EBCL)
3.2.1 Patterned SAM initiators. While in ‘‘lift-off’’ EBL,
the surface materials contrast was used to realize patterned
brushes, electron beam chemical lithography (EBCL) as developed
by Eck et al.
53
allows the introduction of the chemical contrast
within the SAM itself and thus avoids overlaying topographical
features. Electron irradiation of 4-substituted aromatic SAMs
results in a lateral crosslinking of the aryl mesogens along
with a selective reduction of i.e. a terminal nitro to an amino
group or sulfonic acid to a thiol.
54
Advantageously, the lateral
electron-induced conversion of 4-nitro-1,10-biphenyl-4-thiol
(NBT) SAMs to crosslinked 4-amino-1,10-biphenyl-4-thiol
further stabilizes the monolayer by the lateral crosslinking
itself as well as by the multitude of surface attachment points
of the ‘‘polymerized’’ SAM in the irradiated area. As the
chemical conversion of the nitro to the amino group is limited
to areas irradiated by electrons, the technique is referred to as
EBCL. These amine terminated organic nanostructures could
be used as templates for SIP using a surface-bound initiator to
yield densely grafted polymer brush nanopatterns. Go
¨
lzha
¨
user
and Jordan et al.
19,20
first demonstrated the fabrication of
sub-50 nm polymer brush nanopatterns by combining top-down
EBCL with the bottom-up self-assembly of monolayers and SIP.
SAMs of NBT were patterned by EBCL followed by diazotization
and coupling of methylmalonodinitrile to result in well defined
areas of crosslinked surface-bond asymmetric biphenyl/
malonodinitril azo-initiator suitable for thermal as well as photo
polymerization of a broad variety of vinyl monomers. FR-SIP
by thermal
19
or photopolymerization
20
selectively amplified the
Fig. 4 (A) Stepwise fabrication of patterned PNIPAAM brushes created by EBL and SI-ATRP. (B–C) AFM scans of line patterns of gold,
fabricated by EBL and subsequent PNIPAAM brush grown by SI-ATRP from immobilized thiol initiator on the Au
18
(reproduced with
permission from ref. 18, copyright 2004, Wiley-VCH Verlag GmbH & Co. KGaA).
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Frequently asked questions

Q1. What are the future works mentioned in the paper "Rsc_cs_c2cs15225h 1..17" ?

As various lithographic techniques matured and the methods and possibilities of SIP are rapidly developing, the authors will soon see an increase of complexity in terms of the morphology ( in 2D and 3D ) as well as in the chemistry of the surface and the polymer brush. A solution to this might be the use of the chemical contrast of the substrate surface itself that can be created by standard lithographic techniques with a sacrificial layer ( e. g. a photoresist ) or by area-selective deposition such as carbon templating ( CT ). Because both systems are very robust, can be prepared with a high aspect ratio with macroscopic lateral dimensions and nanoscopic thicknesses, these systems are promising candidates for the development of tether-free fast responding micro- or nanochemomechanical systems ( M/NCMS ) that will challenge existing microelectromechanical systems ( MEMS ). Given the higher chemical and physical contrast along with the multiplicity of chemical functions per unit area for a polymer brush as compared to monolayer systems, potential fields for the application of patterned polymer brushes are i. e. in biomedicine for directing and control of protein adsorption and cell adhesion ( non-fouling surfaces ) as well as the use as ( massive parallel ) sensors and actuators in e. g. analytical devices for combinatorial techniques. 

Q2. What contributions have the authors mentioned in the paper "Rsc_cs_c2cs15225h 1..17" ?

This critical review summarizes recent developments in the fabrication of patterned polymer brushes.