Patterned polymer brushes.
Summary (5 min read)
- 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.
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Frequently Asked Questions (2)
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.