Encapsulation is of key importance to improve the stability and lifetime of organic conductors and devices, mainly in applications such as flexible electrodes or organic photovoltaics (OPV). Here, a single-layer conformal encapsulation method is demonstrated via initiated chemical vapor deposition (iCVD) for organic conductor, poly (3,4-ethylenedioxythiophene) (PEDOT). The accelerated degradation tests at 100 °C show that the conductivity of encapsulated PEDOT can be retained up to 17 times longer than that of the unencapsulated counterpart. PEDOT degradation and encapsulation mechanisms are also discussed. Furthermore, the versatility of the iCVD encapsulation on a top-illuminated OPV architecture that can be used to produce low-cost photovoltaics devices on various unconventional substrates (e.g., paper) is demonstrated. Unlike previous approaches of using solely water/oxygen barriers, the encapsulation effect of polymer films on OPV devices is improved by a thin layer capping of evaporated UV-screening material, Cerium(IV) oxide (CeO₂) over the iCVD polymer layer. This bilayer encapsulation strategy efficiently slows the degradation of OPVs and represents a new method to encapsulate OPV and other organic devices.

Highoptical transmittance conjugated-polymers with electrical conductivity are garnering much attention for the applications in organic optoelectronic devices including organic field-effect-transistors and solar cells. Polymers based on PEDOT are particularly promising candidates with high conductivity, uniform surface planarity and excellent ductility. In this work, homopolymer PEDOT deposited using oxidative chemical-vapor-deposition(oCVD) show the maximum conductivity of ≈3500 S/cm. However, their utility is limited due to the relatively low transmittance and abrupt decrease near the red edge in the visible regime. Here, the significantly improved optical properties achieved via tuning the bandgap of cross-linked PEDOT copolymers using oCVD, offering a single-step process for the synthesis and deposition of copolymer films, is reported. The cross-linking monomers of biphenyl or anthracene are simultaneously evaporated with EDOT monomer and an oxidant(FeCl3) during the deposition. Poly(anthracene-co-EDOT)[p(ANTH-co-EDOT)] shows the superior transmittance (≈93%) to homopolymer PEDOT (≈80%) and poly(biphenyl-co-EDOT)[p(BPH-co-EDOT)] (≈88%). Additionally, copolymers show no transmission decay in the red edge regime unlike homopolymer PEDOT that presents an abrupt transmission falloff. An improvement in optical transmittance is in agreement with an increase in bandgap of materials (p(ANTH-co-EDOT), ≈2.3eV vs PEDOT, ≈1.8 eV). oCVD-processed bandgap-tunable PEDOT copolymers with enhanced transmittance may, therefore, have applications in organic optoelectronic devices that require high optical transparency.

The formation and accumulation of clathrate hydrates inside oil and gas pipelines cause severe problems in deep-sea oil/gas operations. In the present work, durable and mechanically robust bilayer poly-divinyl benzene/poly(perfluorodecylacrylate) coatings are developed using initiated chemical vapor deposition (iCVD) to reduce the adhesion strength of hydrates to underlying substrates (silicon and steel). Tetrahydrofuran (THF) dissolved in water with a wt% concentration of 0–70 is used to study the formation of hydrates and their adhesion strength. Goniometric measurements of the THF–water droplets on the substrates exhibit a reduction in advancing and receding contact angles with an increase in the THF concentration. The strength of hydrate adhesion experiences a tenfold reduction when substrates are coated with these iCVD polymers: from 1050 ± 250 kPa on bare silicon to 128 ± 100 kPa on coated silicon and from 1130 ± 185 kPa on bare steel to 153 ± 86 kPa on coated steel. The impact of subcooling temperature and time on the adhesion strength of hydrate on substrates is also studied. The results of this work suggest that the THF–water mixture repellency of a given substrate can be utilized to assess its hydrate-phobic behavior; hence, it opens a pathway for studying hydrate-phobicity.

The transparent conductingpoly(3,4-ethylenedioxythiophene) (PEDOT) is of interest for various optoelectronic device applications. Here, the conductivity stability of PEDOT processed using oxidative chemical-vapor-deposition (oCVD) with FeCl₃ as an oxidant is primarily dominated by the change in carrier density when aged in air. To establish the mechanism for the conductivity decrease, the changes in carrier density and carrier mobility of PEDOT films are separately monitored using an AC Hall Effect measurement system. The measured electrical properties reveal that a decrease in carrier density dominates the conductivity decrease during annealing. X-ray diffraction analysis made on the HBr- and MeOH-rinsed PEDOT samples identifies the Fe-related dedoping phase of Fe(OH)₂ and provides the dedoping mechanism. The carrier transport study demonstrates heavily doped oCVD PEDOT with the carrier density higher than ~10²⁰ cm^–3, and in this regime, an increase in carrier density yields lower carrier mobility which shows that the carrier transport is governed by the ionized impurity scattering mechanism due to increased dopant counter-anions. These findings of the mechanisms for PEDOT conductivity decrease and carrier transport behavior may be important to organic optoelectronic device applications that show a strong effect of air-exposure and low-temperature annealing on the device stability and performance.

Well-adhered, conformal, thin (<100 nm) coatings can easily be obtained by chemical vapor deposition (CVD) for a variety of technological applications. Room temperature modification with functional polymers can be achieved on virtually any substrate: organic, inorganic, rigid, flexible, planar, three-dimensional, dense, or porous. In CVD polymerization, the monomer(s) are delivered to the surface through the vapor phase and then undergo simultaneous polymerization and thin film formation. By eliminating the need to dissolve macromolecules, CVD enables insoluble polymers to be coated and prevents solvent damage to the substrate. CVD film growth proceeds from the substrate up, allowing for interfacial engineering, real-time monitoring, and thickness control. Initiated-CVD shows successful results in terms of rationally designed micro- and nanoengineered materials to control molecular interactions at material surfaces. The success of oxidative-CVD is mainly demonstrated for the deposition of organic conducting and semiconducting polymers.

Chemical vapor deposition (CVD) methods are a powerful technology for engineering surfaces. When CVD is combined with the richness of organic chemistry, the resulting polymeric coatings, deposited without solvents, represent an enabling technology in many different fields of application. This article focuses on initiated chemical vapor deposition (iCVD), a new technique that utilizes benign reaction conditions to yield conformal and functional polymer thin films. The latest achievements in coating surfaces and 3D substrates with functional materials, and the use of the technique for biotechnology and selective permeation applications are reviewed, and future directions for iCVD technology are discussed.

Organic photovoltaics devices typically utilize illumination through a transparent substrate, such as glass or an optically clear plastic. Utilization of opaque substrates, including low cost foils, papers, and textiles, requires architectures that instead allow illumination through the top of the device. Here, we demonstrate top-illuminated organic photovoltaics, employing a dry vapor-printed poly(3,4-ethylenedioxythiophene) (PEDOT) polymer anode deposited by oxidative chemical vapor deposition (oCVD) on top of a small-molecule organic heterojunction based on vacuum-evaporated tetraphenyldibenzoperiflanthene (DBP) and C₆₀ heterojunctions. Application of a molybdenum trioxide (MoO₃) buffer layer prior to oCVD deposition increases the device photocurrent nearly 10 times by preventing oxidation of the underlying photoactive DBP electron donor layer during the oCVD PEDOT deposition, and resulting in power conversion efficiencies of up to 2.8% for the top-illuminated, ITO-free devices, approximately 75% that of the conventional cell architecture with indium-tin oxide (ITO) transparent anode (3.7%). Finally, we demonstrate the broad applicability of this architecture by fabricating devices on a variety of opaque surfaces, including common paper products with over 2.0% power conversion efficiency, the highest to date on such fiber-based substrates.

Preferred crystallographic orientation (texture) in thin films of technologically important materials frequently has a strong effect on the properties of these films and is important for stable surface properties. The deposition of organized molecular films of a poly-perfluorodecylacrylate, poly-(1H,1H,2H,2H-perfluorodecyl acrylate) (p-PFDA), by initiated chemical vapor deposition (iCVD) is described. The tendency of p-PFDA to crystallize in a smectic B phase has been reported in films prepared from solution but not for those using a CVD technique. The degree of crystallinity and the preferred orientation of the perfluoro side chains, either parallel or perpendicular to the surface, are controlled by tuning the CVD process parameters (i.e., initiator to monomer flow rate ratio, filament temperature, and substrate temperature). Films with no observable X-ray diffraction patterns are also achieved. The observed differences in crystal texture strongly impact the observed water contact angles (150° to 130°, advancing) and corresponding hysteresis behavior. Low hysteresis (<7°) is associated with high crystallinity, particularly when the orientation of the crystallites resulted in the perfluoro side groups being oriented parallel to the surface. The latter texture resulted in smoother film than the texture with the chains oriented perpendicular to the surface and this can be very advantageous for applications in which relatively smooth but still crystalline films are needed.

Since their discovery, electrically conductive polymers have gained immense interest both in the fields of basic and applied research. Despite their vast potential in the fabrication of efficient, flexible, and low-cost electronic and optoelectronic devices, they are often difficult to process by wet-chemical methods due to their very low to poor solubility in organic solvents. The use of vapor-based synthetic routes, in which conductive polymers can be synthesized and deposited as a thin film directly on a substrate from the vapor phase, provides many unique advantages. This article discusses oxidative vapor deposition processes, primarily vapor phase polymerization and oxidative chemical vapor deposition, of conjugated polymers and their applications. The mild operating conditions (near room temperature processing) allow conformal and functional coatings of conjugated polymers on delicate substrates. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Fabrication of a chemiresistive biosensor for detection of biomolecules is demonstrated on a high surface area, flexible electro-spun nylon fiber mat. For the first time, the –OH functionalized conducting copolymer of 3,4-ethylenedioxythiophene (EDOT) and 3-thiopheneethanol (3-TE) is synthesized and conformally deposited on the electro-spun mats by oxidative chemical vapor deposition (oCVD). The free –OH functional groups of the copolymer are available for immobilization of analyte specific biomolecules. Here, avidin and biotin molecules are employed as the analyte-specific molecule and analyte respectively for their high specificity to each other. The sensitivities of avidin immobilized conducting copolymer on electro-spun mats are tested against micro-molar to nano-molar concentrations of biotin in aqueous solutions. Application of electro-spun fiber mat in this case enhances the sensor response 6 times when compared to a flat substrate and also significantly lowers the response time. In addition to the experimental studies, current work also includes modeling of the kinetics of the change of response for the biotin-avidin interactions as a function of time. Most importantly, this fabrication technique promises an extremely sensitive and field deployable method for the detection of other biomolecules, for example, food pathogens.

Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric films directly on the surface of a substrate. CVD polymers are desirable as conformal surface modification layers exhibiting strong retention of organic functional groups, and, in some cases, are responsive to external stimuli. Traditional wet-chemical chain- and step-growth mechanisms guide the development of new heterogeneous CVD polymerization techniques. Commonality with inorganic CVD methods facilitates the fabrication of hybrid devices. CVD polymers bridge microfabrication technology with chemical, biological, and nanoparticle systems and assembly. Robust interfaces can be achieved through covalent grafting enabling high-resolution (60 nm) patterning, even on flexible substrates. Utilizing only low-energy input to drive selective chemistry, modest vacuum, and room-temperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductive polymers, and controllably crosslinked networks and for the potential to reduce environmental, health, and safety impacts associated with solvents. Quantitative models aid the development of large-area and roll-to-roll CVD polymer reactors. Relevant background, fundamental principles, and selected applications are reviewed.

Simultaneous improvement of mechanical properties and lowering of the dielectric constant occur when films grown from the cyclic monomer tetravinyltetramethylcyclotetrasiloxane (V4D4) via initiated chemical vapor deposition (iCVD) are thermally cured in air. Clear signatures from silsesquioxane cage structures in the annealed films appear in the Fourier transform IR (1140 cm⁻¹) and Raman (1117 cm⁻¹) spectra. The iCVD method consumes an order of magnitude lower power density than the traditional plasma-enhanced CVD, thus preserving the precursor's delicate ring structure and organic substituents in the as-deposited films. The high degree of structural retention in the as-deposited film allows for the beneficial formation of intrinsically porous silsesquioxane cages upon annealing in air. Complete oxidation of the silicon creates ‘Q’ groups, which impart greater hardness and modulus to the films by increasing the average connectivity number of the film matrix beyond the percolation of rigidity. The removal of labile hydrocarbon moieties allows for the oxidation of the as-deposited film while simultaneously inducing porosity. This combination of events avoids the typical trade-off between improved mechanical properties and higher dielectric constants. Films annealed at 410 °C have a dielectric constant of 2.15, and a hardness and modulus of 0.78 and 5.4 GPa, respectively. The solvent-less and low-energy nature of iCVD make it attractive from an environmental safety and health perspective.

A new nanoscale sensing concept for the detection of nitroaromatic explosives is described. The design consists of nitroaromatic-selective polymeric layers deposited inside microfabricated trenches. As the layers are exposed to nitroaromatic vapors, they swell and contact each other to close an electrical circuit. The nitroaromatic selective polymer, poly(4-vinylpyridine) (P4VP), is deposited in the trenches using initiated chemical vapor deposition (iCVD). P4VP is characterized for the first time as a selective layer for the absorption of nitroaromatic vapors. The Flory–Huggins equation is used to model the swelling response to nitroaromatic vapors. The Flory–Huggins interaction parameter for the P4VP–nitrobenzene system at 40 °C is 0.71 and 0.25 for P4VP–4-nitrotoluene at 60 °C. Sensing of nitrobenzene vapors is demonstrated in a prototype device, while techniques to improve the performance of the design in terms of response time and sensitivities are described. Modeling shows that concentration and mass limits of detection of 0.95 ppb and 3 fg, respectively, can be achieved.

In this work we demonstrate that the concentration of vinyl bonds present in monomers adsorbed onto the surface determines the degree of conformality achieved by polymeric films grown over substrate topology by initiated (i)CVD. For microtrenches of varying aspect ratio, a ballistic model correlates the observed step coverage to a range of sticking coefficients of the initiating radicals, γ, between 0.01 and 0.05. Increasing the surface concentration of the monomer results in higher values of γ. In the deposition regime studied, γ is independent of the concentration of radical initiators. Furthermore, γ is insensitive to substrate temperature if the surface concentration of monomer is fixed. Using this knowledge, conformal coverage on microtrenches is demonstrated from both ethylene glycol diacrylate (EGDA) and neopentyl methacrylate (npMA). Additionally, the conformality permits templating of nanotubes of the mechanically robust and solvent-resistant iCVD-produced p(EGDA) crosslinked polymer.

This paper reports the design, synthesis and characterization of thin films as a platform for studying the separate influences of physical and chemical cues of a matrix on the adhesion, growth and final phenotype of cells. Independent control of the physical and chemical properties of functionalized, swellable hydrogel thin films is achieved using initiated chemical vapor deposition (iCVD). The systematic variation in crosslink density is demonstrated to control the swelling ability of the iCVD hydrogel films based on 2-hydroxyethyl methacrylate (HEMA). At the same time, the incorporation of controllable concentrations of the active ester pentafluorophenyl methacrylate (PFM) allows easy immobilization of aminated bioactive motifs, such as bioactive peptides. Initial cell culture results with human umbilical vein endothelial cells (HUVEC) indicate that the strategy of using PFM to immobilize a cell-adhesion peptide motif onto the hydrogel layers promotes proper HUVEC growth and enhances their phenotype.

Vapor based polymer deposition methods have evolved to address the limitations of solution polymerization. However this field of vapor based polymerization suffers from varied nomenclature and would benefit from a review article that brought together the different approaches to vapor based polymerization with an effort to present existing nomenclature, clearly identify the various approaches within this field and summarize recent results. This review article is written with the purpose of compiling recent advances in vapor based deposition methods of polymers with a focus on applications that will continue to drive research in this field. We hope that this article encourages researchers engineering novel polymer applications to give vapor based polymerization a serious consideration apart from solution polymerization methods.

Initiated (i)CVD is used to deposit thin films of poly(cyclohexylmethacrylate) (pCHMA) in microtrenches of depth 7 µm and widths 1-5 µm. By changing the fractional saturation of the monomer vapor, step coverage of 0.85 is achieved for the highest aspect ratio trench studied while maintaining a deposition rate of 15 nm min⁻¹. An analytical model for determining the sticking probability of the initiating radical (CH₃)₃CO· is developed, and is experimentally shown to be a function of the fractional saturation of the monomer vapor. For the conditions studied, the sticking probability is in the range 1.1 × 10⁻²–5.0 × 10⁻². These results suggest that iCVD proceeds via the reaction of a vapor-phase initiating radical with a surface-adsorbed monomer.