3.1. Overview and Mechanism
Initiated CVD (iCVD) is a free-radical polymerization method. The initiator and monomer species enter the iCVD chamber as vapors. In analogy to solution-phase synthesis, the iCVD initiator decomposes to form radical species (Figure 1b) that then activate the chain growth polymerization of the monomers. Initiators successful used for iCVD include perfluorooctane sulfonyl fluoride (PFOS),50 perfluoro-1-butanesulfonyl fluoride (PBSF),51 triethylamine (TEA),52 tert-butyl peroxide (TBPO),53 tert-amyl peroxide (TAPO),54 and tert-butyl peroxybenzoate (TBPOP).55 These initiators contain a labile bond, such as the O-O bond in peroxides, which can be readily cleaved. Ideally, at the relatively mild conditions required to decompose the initiator, the monomers fully retain their pendent functional groups. The result is a surface of well-defined chemical composition having a high density of functionalities. Surface moieties such as perfluoroalkanes and hydroxyl enable systematic adjustment of the surface energy. Reactive groups such as amine, epoxy, carboxylic acid, and propargyl allow for functional attachment of molecules, cells, and nanoparticles to the surface. The full retention of the organic functionality also enables the synthesis of responsive surfaces.
Several options have been demonstrated for inducing initiator decomposition. Using thermal decomposition over heated filament array is denoted as iCVD.56 Alternatively, the decomposition can be achieved by a low energy plasma discharge, termed, initiated plasma enhanced CVD (iPECVD),57 or by UV light, termed photo initiated CVD (piCVD).58 A variant of piCVD is grafted CVD (gCVD), in which the benzophenone (BP), when photo-decomposed, creates radical sites directly on the surface of the substrate.59 The reaction of the surface radicals with the monomer species results in covalently tethered chains.
All these processes require only modest energy input and operate at low surface temperatures (ca. 20–70 °C). Keeping the surface at low temperature, promotes the adsorption of the monomers. Even though the gas phase monomer concentration is quite dilute, the liquid-like monomer concentration in the surface layer enables rapid chain growth. Thus, despite the benign reaction conditions of iCVD, high deposition rates can often be achieved (>200 nm/min). High deposition rates and efficient consumption of the precursors are essential for industrial applications. Understanding the deposition mechanism is therefore needed to predict the deposition conditions which yield high deposition rate and is also useful in optimizing the iCVD process for producing conformal coatings over structured surfaces.
The mechanism of iCVD polymerization has been well documented in literature and it has been demonstrated to follow the same steps of conventional free-radical polymerizations: initiation, propagation, and termination.54, 60, 61, 62 However, while in the liquid phase all of the reaction steps occur at a single temperature, for iCVD some reactions steps take place homogeneously in gas phase at a temperature, which can reach that of the filament, while other reaction steps take place heterogeneously on the much cooler surface of the substrate. The first step involves the thermal fragmentation of the initiator:
The fragmentation takes place in the gas phase, at or near the heated filament. The initiator and its fragments are typically quite volatile and therefore have only limited adsorption on the surface. However, the monomers are typically much less volatile and can readily reach concentrations on the surface, which represent significant fraction of a monolayer and in some cases even exceed monolayer coverage.
A radical impinging on the site of absorbed monomer can undergo a heterogeneous reaction to create a surface radical:
The product of reaction 3 readily reacts with other adsorbed monomers via a propagation step:
When the kinetics of reactions involving the monomer's vinyl bond are the rate-limiting step, the observed iCVD deposition rate increases as substrate temperature increases. Conversely, the iCVD growth decreases when the rate-limiting step is the adsorption of monomer to the surface. This adsorption limited regime is often observed for acrylate monomers, which generally have a high propagation rate constant,61 while the reaction limited regime is typically observed for less reactive vinyl bonds, such as those substituted on to organosilicones.63 The iCVD polymerization can terminate when the radical ends are capped by reaction with other growing polymeric chains or with initiator radicals.
Numerous studies in literature have demonstrated that substrate temperature,61, 63 monomer flow rate,64 and filament temperature61 all impact deposition kinetics. However, the ratio between the monomer partial pressure (PM) and the saturation pressure at the substrate temperature (Psat) is often the dominant parameter.54, 64 Quartz crystal microbalance studies reveal that PM/Psat is directly correlated with the concentration of monomer adsorbed on the surface (Figure 7a) through the Brunauer–Emmett–Teller adsorption isotherm:60
where . is the total adsorbed volume; Vml is the monolayer-adsorbed volume. ΔHdes is the enthalpy of desorption and ΔHvap the enthalpy of evaporation.
Figure 7. Kinetic studies on the iCVD process. a) Quartz microbalance measurement of the adsorbed volume and calculated areal concentration as a function of the PM/Psat. b) Arrhenius plot of the deposition rate of p(EGDA) as a function of the inverse of the gas temperature. At low filament temperature, the apparent activation energy (166 kJ/mol) has been calculated from the slope of the linear regression of the data. c) FTIR and C 1s XPS spectra of the PVP deposited by iCVD and the standard commercial polymer. The iCVD polymer is nearly spectroscopically identical to the commercial one. a) Reproduced with permission.54 Copyright 2006, American Chemical Society. b) Reproduced with permission.61 Copyright 2009, American Vacuum Society. c) Reprdouced with permission.62 Copyright 2006, American Chemical Society.
Download figure to PowerPoint
In iCVD kinetics studies the enthalpy of evaporation, ΔHvap, determined falls in the range of 20–80 kJ/mol, which is characteristic of physisorption of small molecules on surfaces through van der Waals interactions. The monomer adsorption is promoted by low substrate temperature. Additionally, the surface concentration of monomer determines the sticking probability of the initiator radicals.64
The surface polymerization has been proposed to follow the Eley–Rideal mechanism in which the deposition rate depends on the surface coverage of the monomer (θM) and the gas-phase concentration of the radicals, since the radicals have much higher volatility and thus much lower adsorption on the surface.64
Equation 6 is one of the main kinetic equation describing the radical polymerization growth rate together with the kinetic equations that give the chain length (υ) and molecular weight (Mn):
where Ri is the initiation rate, characterized by the rate constant ki. The number-average degree of polymerization, (Xn= 2υ/(2– a)) is used to calculate the number-average molecular weight, Mn. The parameter a reflects the fraction of polymer radicals that terminate by coupling, (1– a) being the fraction terminating by disproportionation, while MWM is the molar mass of the polymer repeat unit.
Through a multi-response parameter estimation procedure Lau and Gleason60 developed in 2006 a quantitative model which related the kinetic equations (Equation 6 and 7) with the PM/Psat. The model uses the assumptions that the initiator is decomposed in the gas-phase at Tfil and the other reactions (initiation, Equation 2; propagation, Equation 3 and 4; and termination, Equation 9) occur at the surface at Tsub (substrate temperature). The termination reactions considered in the model were bimolecular chain termination through coupling or disproportionation, primary radical termination through the attack of a primary radical on a polymer radical or primary radicals recombination, respectively:
Chain transfer processes were not considered as termination reaction because under vacuum many of them are either absent or negligible (e.g., there is no transfer to solvent). The modelization of the dependence of the film growth rate and the molecular weight has been largely used to predict the feasibility of some new process or the scalability in large area reactors.
Also, the filament temperature plays a fundamental role in the process kinetics. While it does not influence the PM/Psat ratio on the surface, it does influence the concentration of radicals created, and hence the rate at which chains are initiated (Equation 6). Figure 7b shows the Arrhenius plot of the deposition rate as a function of the filament temperature.61 Two regimes can be observed. At low Tfil, the film growth rate increases rapidly with increasing filament temperature, characteristic of a reaction-kinetics-limited-process. At high Tfil, the deposition rate is less influenced by the changes in the temperatures, which is characteristic of a mass-transfer-limited-regime. In the kinetics-limited regime, the apparent activation energy calculated experimentally from the Arrhenius plot is 166 ± 5 kJmol−1, which is in good agreement with the activation energy required to decompose the TBPO molecule (163.6 kJ mol−1). In the mass-transfer-regime, the filament temperature is high enough to efficiently decompose the initiator so the deposition kinetics is instead dominated by the diffusion of the radicals from the gas phase to the substrate surface. Decreasing the mass transfer resistance by increasing the gas flow rate increases the deposition rate in this regime (Figure 7b).
The copolymerization kinetics follows the same rules as in conventional free-radical copolymerization reactions. The copolymer composition will depend on the monomer ratio at the surface. This surface ratio differs from the ratio in the feed gases when the monomers differ in volatility (e.g., have different saturation pressures). Additionally, the tendency of two monomers to copolymerize was quantified through the reactivity ratio.65 The reactivity ratios describe the propensity of the propagating species to add a homo-monomer or the other monomer. In other words, considering a growing copolymer of generic monomers A and B, terminating with a A unit, then the reactivity ratio rA can be defined as the ratio between kA and kB according to the following reactions:
Analogously, the reactivity ratio rB for the growing copolymer terminating with a B unit can be identified. The copolymer composition is determined by the Fineman–Ross equation66 in the form:
where f is the monomer fraction in the gas feed, and F the monomer fraction at the surface.
3.2. Retention of Organic Functional Groups
Many thin film applications benefit from a rationally designed chemistry for controlling the properties and molecular interactions at the surface. For instance, in the biomedical field, the chemistry of the surface, as quantified by density of functional groups, is essential for controlling protein bonding or the attachment and growth of cells.12, 67, 68 Additionally, the dynamic response of stimuli-responsive materials, that is to say, materials that change their properties following an external stimulus, also depends strongly on the density of functional groups on the surface.69, 70, 71
For these reasons, it is very important to chemically design the deposition process, choosing the right monomer with the desired functional groups. Retaining these organic functional groups is not trivial task for most CVD processes. In general, it has been observed that in order to retain the functional groups of the monomer, the deposition rate drops down to only a few nm/min. This is especially true when the monomer is the species that is fragmented to create active sites for polymerization, as in plasma enhanced CVD (PECVD), where labile functional groups are easily cleaved off.72 In contrast, the monomer remains intact during iCVD, and thus, fully retains the organic moieties desired for surface design.
The iCVD method is a platform technology for yielding functional polymers at high deposition rates (>200 nm/min). Successful examples of polymers deposited by iCVD are poly(aminostyrene) (PAS) which displays a high density of functionalizable–NH2 groups,4 poly(N-isopropylacrylamide) (PNIPAAm) whose temperature-sensitive hydrophobicity is due to the presence of amide and isopropyl groups,73 poly(glycidyl methacrylate) (PGMA) with reactive epoxy groups53, 74 or poly(hydroxyethyl methacrylate) (PHEMA) hydrogels, with hydrophilic hydroxyl moieties.67, 75, 76 PGMA has been deposited also at extraordinary high deposition rates (600 nm/min) by iCVD from supersaturated monomer vapor (PM/Psat > 1).77 The possibility of obtaining polymer thin films spectroscopically identical to their bulk counterpart, makes iCVD a competitor with conventional wet processes (e.g., spin coating, dip casting, etc.) for thin-film applications. The iCVD approach couples the versatility of organic chemistry for the synthesis of polymers with the advantages of dry processing, which is highly beneficial for thin film technologies and device fabrication.
High functional group retention by iCVD has been demonstrated with tens of monomers, including iCVD p(1-vinyl-2- pyrrollidone) (PVP).78 The retention of the pyrrolidone functionality is important to achieve the hydrophilicity, biocompatibility, and antifouling properties, characteristic of this polymer. Figure 7c compares iCVD PVP to a conventionally polymerized PVP standard (PVP360), dissolved in water and cast onto a Si substrate. The Fourier transform infrared (FTIR) spectroscopy, carbon 1s X-ray photoelectron spectroscopy (XPS), and 1H NMR spectroscopy (not shown), all confirm the similar chemical structure of the films produced by both methods. Vinyl bonds are not detected in the FTIR spectra, confirming that the iCVD polymerization indeed occurs through the saturation of all the vinyl bonds of the monomer.
The functional group retention has also been very beneficial to create nanoadhesive trenches.4 Prototype microfluidic structures were fabricated by reacting one side of the device coated with PAS, containing–NH2 groups and the other side with PGMA, with epoxy groups. The high amine density of the iCVD films enabled the formation of a robust nanoadhesive, which was orders of magnitude more robust than the counterpart obtained by depositing the polymers by PECVD with a lower density of functional groups at the surface.
High densities of functional groups are also important for creating a platform for further post-deposition reactions. Zwitterionic coatings were obtained by reacting the fully retained tertiary amine groups of the polymer deposited by iCVD with 1,3-propane sultone.79 A high density of zwitterionic moieties was demonstrated by both depth profiling and angle-resolved XPS measurements in the top ca. 3 nm of the film. Such a high density of zwitterionic groups is difficult to obtain with other techniques with comparable processing time. The zwitterionic functionalized surfaces exhibited excellent anti-fouling properties against protein, carbohydrate, and bacterial adhesion.
The fragmentation of the initiator instead of the monomer molecule is the key for functional group retention. The same principle has been used in the iPECVD process.57 During iPECVD, the chamber was fed also with TBPO, contrarily to conventional PECVD processes where any initiator is used. The presence of the initiator allowed a sufficient density of reactive radicals to be obtained at very low plasma power density. Under these conditions the monomer fragmentation was minimal and therefore the monomer structure retention was enhanced compared to other PECVD processes, without compromising the deposition rate.
The ability to pattern functional iCVD polymers is necessary to enable their incorporation in applications such as sensors, optical devices, and tools for biological research. Several patterning techniques have been demonstrated to date, including methods based on photo- and electron beam-lithography, imprinting, and colloidal lithography. Mao and Gleason demonstrated high-resolution, positive-tone patterning of annealed iCVD poly(methyl α-chloroacrylate-co-methacrylic acid) via electron beam irradiation (Figure 12a).121 iCVD PGMA has also been used as a positive deep-UV resist and negative electron-beam lithography resist;122, 123 the ability to conformally coat nonplanar surfaces123 and use an all-dry, supercritical CO2 developing process122 makes this resist applicable for a range of applications.
Figure 12. a) Positive-tone features patterned in a polyacrylic iCVD thin film using electron-beam lithography. b) 4 mm-diameter glass rod patterned by exposing diacetylene-functionalized iCVD poly(4-vinylpyridine) (P4VP) through a flexible mask. c) iCVD P4VP features patterned using a chemically non-selective lift-off technique. d) iCVD bifunctional surface exhibiting independent chemical and topological contrast. e) Nanodomains of iCVD and PECVD films patterned using capillary force lithography; light and dark-colored regions contain orthogonal acetylene and amine functionalities, respectively. f) Arrays of self-assembled microstructures created by coating elastomeric pillars with hydrophilic iCVD polymer and submerging in water. g) Close-up of a microstructure depicted in (f); solvent bonds form between the pillars, causing them to remain assembled after the system is dried. h) By patterning the pillar-coated substrate with regions of hydrophobic polymer, the regions of self-assembly can be controlled. a) Reproduced with permission.121 Copyright 2006, American Chemical Society. b) Reproduced with permission.124 Copyright 2012, Wiley. c) Reproduced with permission.125 Copyright 2013, IEEE. d) Reproduced with permission.128 Copyright 2010, Wiley. e) Reproduced with permission.126 Copyright 2008, American Chemical Society. f–h) Reproduced with permission.127 Copyright 2011, American Chemical Society.
Download figure to PowerPoint
Photochemical patterning of 3D surfaces has recently been demonstrated with various iCVD polymers. Haller et al.101 achieved positive-tone patterning of porous substrates with iCVD poly(o-nitrobenzyl methacrylate) (PoNBMA), a photosensitive polymer that can be developed in a biologically-compatible buffer. High-curvature surfaces were patterned using a resist comprised of conformal iCVD poly(4-vinylpyridine) (P4VP) functionalized with a photoactive diacetylene.124 Coated substrates were covered with flexible masks and exposed at 254 nm, which photopolymerized the diacetylene and rendered the exposed regions insoluble in the developing solvent (Figure 12b). Bifunctional organic surfaces were also patterned; additionally, the iCVD-based resist was used to create metal microstructures via pattern transfer.
Other successful patterning strategies have been based on masking and imprinting techniques. Functional polymer features were obtained by depositing a conformal iCVD film on resist templates defined using electron-beam lithography; the templates were then selectively removed using a non-solvent for the iCVD polymer (Figure 12c).125 This method is independent of the chemical functionality of the iCVD film, allowing for easy exchange of responsive iCVD polymers for device applications. Fabrication of bifunctional surfaces has been demonstrated using a self-aligned, dual-purpose lithographic mask (Figure 12d).128 The resulting surfaces exhibited chemical and topological contrast and were used to achieve confinement of water droplets during microcondensation. Capillary force lithography has also been used to pattern iCVD films.126 By pressing a heated PDMS mold onto a bilayer of highly crosslinked PECVD poly(allylamine) and iCVD polypoly(propargyl methacrylate), Im and coworkers were able to achieve segregation of orthogonal amine and acetylene functionalities with spatial resolution approaching 100 nm (Figure 12e).
Self-assembly interactions were utilized to facilitate controllable adhesion of iCVD-coated surfaces. Chen et al. coated elastomeric pillars with hydrophilic iCVD polymers, which self-assembled into clusters upon submersion in water (Figure 12f,g).127 Solvent bonds formed by interpenetration of polymer chains overcome the elastic restoring force, rendering the clusters stable upon drying. By patterning some of the pillars with hydrophobic PoNBMA, the authors were able to direct self-assembly and control the location of pillar collapse (Figure 12h).
Functional, patterned iCVD polymers have been incorporated in a variety of applications. Kwong and Gupta integrated multiple unit operations into a single, paper-based microfluidic device.129 Acidic iCVD poly(methacrylic acid) and basic poly(dimethylaminoethyl methacrylate) were deposited on chromatography paper and used as ion-exchange coatings for analyte separation. Patterned iCVD PoNBMA was used as a UV-responsive switch that changed hydrophobicity upon exposure, allowing passage of the analytes into the separation zone (Figure 13a). The device was successfully used to separate mixtures of model anionic and cationic compounds (Figure 13b–d). Haller et al. also patterned microfluidic channels on paper using hydrophobic, photoresponsive PoNBMA (Figure 13e).101
Figure 13. a) Schematic of a paper-based microfluidic device consisting of a separation zone coated with acidic or basic iCVD polymer and a UV-responsive switch consisting of a patterned region of poly(o-nitrobenzyl methacrylate). b–d) Separation of toluidine blue O (cationic analyte) and ponceau S (anionic analyte) on an uncoated paper microfluidic channel (b), a channel coated with acidic iCVD poly(methacrylic acid) (c), and a channel coated with basic iCVD poly(dimethylaminoethyl methacrylate) (d). e) Microfluidic channels coated on paper using a hydrophobic, photoresponsive iCVD polymer. The channels exhibit excellent retention of aqueous dye solutions. f) Overlaid fluorescence micrographs of iCVD and PECVD orthogonal nanodomains. The click-functionalized red dye is excited at 545 nm and the N-hydroxysuccinimide-functionalized green dye is excited at 491 nm. g,h) Fluorescent micrographs of an iCVD hydrogel covalently functionalized with CdSe/ZnS nanoparticles in the dry state (g) and swollen upon immersion in pH 8 buffer solution (h). a–d) Reproduced with permission.129 Copyright 2012, American Chemical Society. e) Reproduced with permission.101 Copyright 2011, Royal Society of Chemistry. f) Reproduced with permission.126 Copyright 2008, American Chemical Society. g,h) Reproduced with permission.130 Copyright 2009, American Chemical Society.
Download figure to PowerPoint
Patterned iCVD surfaces form excellent substrates for attachment of molecules and nanoparticles. Im and coworkers used surfaces patterned with orthogonal amine and acetylene nanodomains (Figure 12e) to selectively self-sort fluorescent dyes via a one-pot, biocompatible click/N-hydroxysuccinimide functionalization step (Figure 13f).126 Tenhaeff and Gleason synthesized a highly swellable, pH-responsive hydrogel, polymaleic anhydride-co-dimethylacrylamide-co-di(ethylene glycol) di(vinyl ether)] (PMaDD), for use in composite separation membranes.130 The retention of functional moieties and extreme swelling response of the films were confirmed by covalently attaching CdSe/ZnS quantum dots to patterned PMaDD and imaging the films upon exposure to a pH 8 buffer solution (Figure 13g,h).
iCVD films can also be used to inhibit molecular attachment—for example, in the case of anti-fouling coatings. As discussed in Section 3.3.3, iCVD PEO films were prepared via a ring-opening cationic polymerization mechanism.104 PEO features were easily patterned using microcontact printing; in addition, grafted and patterned PEO regions illustrated excellent resistance to non-specific protein adsorption, in contrast to control surfaces (Figure 9b).
In the surface modifications of non-planar substrates, the term conformality describes the ability to encapsulate the entire surface topography with a coating of uniform thickness and composition. Conformal coating is desirable since it brings novel surface functionality to substrates without changing their original morphology. The conformal nature of iCVD is quite valuable for producing functional organic coatings over surfaces having complex geometrical features. With rapid development over the past few years, iCVD has been used on many complex substrates. Even the inner surfaces of porous and fibrous substrates can be modified with functional organic materials by iCVD.131, 132, 133, 134
For solution phase processing, undesirable variations in coating thickness can result from de-wetting, liquid thinning, and surface tension effects. In contrast, the conformality of CVD polymers results from the arrival of reactants to the surface by non-line-of-sight vapor phase diffusion under the modest vacuum conditions (typically, 0.1 to 1.0 Torr), combined with the limited probability of the reactants “sticking” to the surface during a single collision. Step coverage, S, the ratio of film thickness at the bottom of the feature to that at the top, is one measure of conformality. An analytic solution for iCVD film growth in a trench feature gives:
where (L/w) is the aspect ratio of the feature and Γ is the sticking coefficient64 This profile is experimentally observed when gas phase diffusion to supply additional monomer to the surface is rapid compared to the rate at which monomer is depleted by the film formation reaction. Since the step coverage is governed mainly by the chemisorption of the radicals onto the adsorbed monomer sites, depositions at low fractional saturation partial pressure ratios (PM/Psat), leads to improved conformality due to lower sticking coefficients. Sticking coefficients in the range of ca. 0.001–0.01 have been observed for iCVD.
The conformality of iCVD has been demonstrated on trench features etched into silicon, with aspect ratios up to 20:1 and on pores in various types of membranes, with aspect ratios up to 400:1. The primary variables that control the conformality of iCVD films, monomer partial pressure and substrate temperature, also control film deposition rate. At the highest deposition rates, >100 nm/min, the step coverage decreases and the thicker coating near the entrance to a cylindrical pore results in a bottleneck profile (i.e., higher film thickness at the pore entrance). However, a high degree of conformality is still achieved at iCVD growth rates of up to 100 nm/min. This rate is quite fast compared to conformal methods for other materials and is also quite reasonable since the electrolyte film thickness of <100 nm are desired.
The limit up to which conformal coverage can be achieved has been determined by analogy to the reaction-diffusion problem commonly encountered for catalysts on porous supports.135 The analytic solution for the dimensionless monomer concentration, ψ, as a function of λ, the dimensionless distance down the pore, depends on the dimensional parameter known as the Thiele modulus, Φ:
The Thiele modulus can be readily calculated from the dimensions of the pore, the diffusivity of the monomer, and the reaction rate. Depending on the deposition conditions, different profiles over 3D-substrates can be obtained.136 This analysis will guide the development of conformal iCVD coating on nanostructured electrodes having different architectures, which give rise to pores of different aspect ratio.
Figure 14a shows the micrograph of a poly(caprolactone) (PCL) nanofiber mat before and after being iCVD coated with poly(perfluoroalkyl ethyl methacrylate) (PPFEMA).3 The conformal coating by the iCVD process maintains the hierarchical nature of the electrospun mat morphology except for a slight increase in the diameter of fibers. The comparison of the XPS survey scans before and after coating (Figure 14b) demonstrates that the fibers were actually coated. Figure 14c displays a poly vinyl alcohol (PVA) mold coated by iCVD PGMA, as the supporting layer, and poly(1H,1H,2H,2H-perfluorodecyl acrylate) p(PFDA), on top of PGMA layer.137 SEM comparison between substrates with and without iCVD coating shows the conformal nature of iCVD process on the inner surfaces of the polymeric mold. The nano-scale inner surfaces features of the PVA mold are exactly repeated by the iCVD deposited films. Additionally, the all-dry nature of the iCVD process avoids the possibility of the solvent swelling or damaging to the substrate, hence ruining the surface topography.
Figure 14. a) SEM images of PCL electrospun mats before and after conformal iCVD coating of PPFEMA (scale bars = 2 μm). b) XPS spectra for samples shown in (a). c) Top view SEM images of PVA mold before and after iCVD coating of PGMA and p(PFDA) (scale bar = 30 nm). The iCVD layer conformally coated inner surfaces of the polymer mold. d) Cross-sectional SEM images of iCVD deposited pCHMA on a trench wafer substrate. e) Cross-sectional SEM images of TiO2 electrodes (4 μm thick) before and after iCVD coating of PHEMA (scale bar = 100 nm), which keeps the morphology of the porous network unchanged. a,b) Reproduced with permission.3 Copyright 2005, American Chemical Society. c) Reproduced with permission.137 Copyright 2012, Elsevier. d) Reproduced with permission.20 Copyright 2011, Wiley. e) Reproduced with permission.138 Copyright 2011, Elsevier.
Download figure to PowerPoint
Figure 14d shows a conformal poly(cyclohexyl methacrylate) (pCHMA) thin films deposited by the iCVD process on a trench wafer.132 This work exhibits the potential for utilizing conformal iCVD coating on microfluidic channels. PDMS-based microfluidic channels provide superior control over mixing than traditional batch reactions.139 These microchannels need to be impermeable to liquids, but PDMS may swell in solutions. Conformal fluoropolymer coatings by iCVD processes can serve as excellent liquid barriers of PDMS to prevent swelling and hydrophobic recovery caused by the low glass transition temperature of PDMS.4, 88, 120, 129, 140–142 Riche et al. demonstrated that conformal fluorocarbon films deposited by iCVD prevent absorption of small molecules and swelling of PDMS in organic solvents.88 This achievement provides a way to synthesize nanomaterials that are difficult to synthesize with conventional batch reactions. Lazarus et al. showed that devices coated in such way were used for more than 24 hours without degradation or delamination.140 Jeong et al. deposited p(PDFA) via iCVD on the PDMS micromold, which substantially reduced the diffusion of oxygen and the swelling caused by organic solvents.141
Conformal iCVD coatings also display excellent pore-filling properties, which is useful for dye-sensitized solar cells (DSSC).138, 143 iCVD polymers can replace the liquid electrolyte used in the cell and enhance its operability and durability. Figure 14e shows pore-filling of a 4 μm thick TiO2 electrode coated with poly(2-hydroxyethyl methacrylate) (PHEMA).143 In this work, Nejati et al. were able to achieve ca. 92–100% pore filling for electrodes as thick as 12 μm.143 Their research also indicated the solid state PHEMA electrolyte has higher efficiency than the liquid electrolyte, and results an enhanced performance of the DSSC.
As a dry mechanism without liquid phase or excipient to produce a conformal polymerization, iCVD also provides the ability to encapsulate fine particles down to nanoscale without challenges of particle agglomeration, toxic solvents, and poor quality control in conventional coatings.144