Polymer Overlayer Research Articles

Modeling Heater Electrodes on Sheet Metal for Arbitrary Temperature Distributions

We present a method for the design of heater electrodes on substrates with high thermal conductivity such as sheet metal. The substrate is covered with a thin polymer insulation layer on both faces, which are, in turn, carrying screen printed, electrically conductive heater electrodes and another protective polymer overlayer. The temperature inside a predetermined optimization area is required to be as uniform as possible, which is desirable for various high-precision heating applications. We alternatively also aim to design a heater structure to create non-uniform temperature distributions such as a temperature dip, a peak or a gradient. These temperature distributions are required for droplet operations like splitting, merging, and moving droplets, subsequently realizing a newly proposed “lab on metal” approach in microfluidics. For this purpose, an optimization algorithm yields the required distances between linear heater electrodes. The resulting electrode distribution was numerically obtained for different optimization areas. Finally we describe the fabrication of a test device and show infrared measurements of the FEM modeled temperature distributions on the experimental realized heaters. We describe the design of heater electrodes, capable of triggering droplet motion on a “Lab-On-Metal”, as an alternative technology in Lab-On-a-Chip applications.

Developing Catalysts Integrated in Gas-Diffusion Electrodes for CO2 Electrolyzers.

ConspectusAs the demand for a carbon-neutral society grows rapidly, research on CO2 electrolysis has become very active. Many catalysts are reported for converting CO2 into value-added products by electrochemical reactions, which have to perform at high Faradaic and energy efficiency to become commercially viable. Various types of CO2 electrolyzers have been used in this effort, such as the H-cell, flow cell, and zero-gap membrane-electrode assembly (MEA) cell. H-cell studies are conducted with electrodes immersed in CO2-saturated electrolyte and have been used to elucidate reaction pathways and kinetic parameters of electrochemical CO2 reduction on many types of catalytic surfaces. From a transport phenomenological perspective, the low solubility and diffusion of CO2 to the electrode surface severely limit the maximum attainable current density, and this metric has been shown to have significant influence on the product spectrum. Flow and MEA cells provide a solution in the form of gas-diffusion electrodes (GDEs) that enable gaseous CO2 to closely reach the catalyst layer and yield record-high current densities. Because GDEs involve a complicated interface consisting of the catalyst surface, gaseous CO2, polymer overlayers, and liquid electrolyte, catalysts with high intrinsic activity might not show high performance in these GDE-based electrolyzers. Catalysts showing low overpotentials at low current densities may suffer from poor electron conductivity and mass transfer limitations at high current densities. Furthermore, the stability of the GDE-based catalysts is often compromised as CO2 electrolysis is pursued with high activity, most notoriously by electrolyte flooding.In this Account, we introduce recent examples where the electrocatalysts were integrated in GDEs, achieving high production rates. The performance of such systems is contingent on both GDE and cell design, and various parameters that affect the cell performance are discussed. Gaseous products, such as carbon monoxide, methane, and ethylene, and liquid products, such as formate and ethanol, have been mainly reported with high partial current density using the flow or MEA cells. Different strategies to this end are described, such as controlling microenvironments by the use of polymers mixed within the catalyst layer or the functionalization of catalyst surfaces with ligands to increase local concentrations of intermediates. Unique CO2 electrolyzer designs are also treated, including the incorporation of light-responsive plasmonic catalysts in the GDE, and combining the electrolyzer with a fermenter utilizing a microbial biocatalyst to synthesize complex multicarbon products. Basic conditions which the catalyst should satisfy to be adapted in the GDEs are listed, and our perspective is provided.

Polymer-Regulated Electrochemical Reduction of CO2 on Ag.

The influence of polymer overlayers on the catalytic activity of Ag for electrochemical CO2 reduction to CO is explored. Polystyrene and poly(4-vinylpyridine) films of varying thicknesses are applied as catalysis-directing overlayers atop Ag electrodes. For polystyrene, substantial suppression of CO2 reduction activity is observed while the hydrogen evolution reaction (HER) increases. The addition of a nitrogen heteroatom into the phenyl groups of polystyrene (e.g., a pyridine ring) results in an increase in the conversion of CO2 to CO and suppression of HER. Block copolymer variants containing both phenyl and pyridyl functionalities exhibit similar activity for CO evolution but appear to suppress HER further than the polymer layer containing only pyridine groups. The size of the blocks for the copolymer influences the catalytic output of the Ag electrode, suggesting that the hierarchical structure that forms in the block copolymer layer plays a role in catalytic activity at the electrode surface. Analysis of the polymer overlayers suggests that polystyrene significantly inhibits all ion transport to the metal electrode, while poly(4-vinylpyridine) enables CO2 transport while modifying the electronics of the Ag active site. Therefore, the engineered application of polymer overlayers, especially those containing heteroatoms, enables new avenues of electrochemical CO2 reduction to be explored.

Long‐Lived Charges in Y6:PM6 Bulk‐Heterojunction Photoanodes with a Polymer Overlayer Improve Photoelectrocatalytic Performance

AbstractPhotogenerating charges with long lifetimes to drive catalysis is challenging in organic semiconductors. Here, the role of a PM6 polymer overlayer on the photoexcited carrier dynamics is investigated in a Y6:PM6 bulk‐heterojunction (BHJ) photoanode undergoing ascorbic acid oxidation. With the additional polymer layer, the hole lifetime is increased in the solid state BHJ film. When the photoanode is electrically coupled to a hydrogen‐evolving platinum cathode, remarkably long‐lived hole polaron states are observed on the timescale of seconds under operational conditions. It is demonstrated that these long‐lived holes enable the organic photoanode with the polymer overlayer to show enhanced ascorbic acid oxidation performance, reaching ≈7 mA cm−2 at 1.23 VRHE without a co‐catalyst. An external quantum efficiency of 18% is observed using 850 nm excitation. It is proposed that the use of an organic overlayer can be an effective design strategy for generating longer charge carrier lifetimes in organic photoanodes for efficient oxidation catalysis.

A Comparative Study of PMETAC-Modified Mesoporous Silica and Titania Thin Films for Molecular Transport Manipulation.

The manipulation and understanding of molecular transport across functionalized nanopores will take us closer to mimicking biological membranes and thus to design high-performance permselective separation systems. In this work, Surface-initiated atom transfer radical polymerization (SI-ATRP) of (2-methacryloyloxy)-ethyltrimethylammonium chloride (METAC) was performed on both mesoporous silica and mesoporous titania thin films. Pores were proven to be filled using ellipsometry and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Furthermore, the employed method leads to a polymer overlayer, whose thickness could be discriminated using a double-layer ellipsometry model. Cyclic voltammetry experiments reveal that the transport of electrochemically active probes is affected by the PMETAC presence, both due to the polymer overlayer and the confined charge of the pore-tethered PMETAC. A more detailed study demonstrates that ion permeability depends on the combined role of the inorganic scaffolds' (titania and silica) surface chemistry and the steric and charge exclusion properties of the polyelectrolyte. Interestingly, highly charged negative walls with positively charged polymers may resemble zwitterionic polymer behavior in confined environments.

Effect of transparent polymer encapsulation overlayers on bending fracture behavior of flexible organic lead halide perovskite thin films

Although halide perovskites have significant potential for flexible devices, the bending mechanical behavior of halide thin films has not been investigated in detail. Herein, we report the quantitative bending fracture behavior of flexible methylammonium lead triiodide (MAPbI 3 ) thin films encapsulated with an overlayer of either polystyrene (PS) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The significantly retarded cracking behavior of the halide films demonstrated that adjusting the polymer overlayers with different thicknesses resulted in substantial mechanical enhancements. For example, the minimum strain for crack initiation was ~1.96% for a highly concentrated PS overlayer with a thickness of ~50 nm, which corresponds to an increase of ~40% relative to the non-encapsulated sample. In addition to quantifying the fracture behavior with fracture energy and film strength, the chemical effects of encapsulation were also investigated in terms of the phase dissolution of the perovskite, which was correlated with changes in the Hall electrical properties. • Bending fracture behavior of polymer-encapsulated perovskite halide films is studied. • Less cracks were observed with the thicker polystyrene overlayer under bending. • A 50 nm-thick polystyrene layer resulted in a 96% improvement of fracture energy. • Phase dissociation with atmospheric exposure was suppressed by polymer-encapsulation. • Hall electrical resistivity was correlative to the level of phase dissociation.

A Dual Functional Polymer Interlayer Enables Near‐Infrared Absorbing Organic Photoanodes for Solar Water Oxidation (Adv. Energy Mater. 18/2022)

Solar Water Oxidation In article number 2103698, James R. Durrant and co-workers design organic bulk-heterojunction photoanodes with a polymer overlayer for solar water oxidation. The additional polymer layer improves underwater operational stability and suppresses recombination losses. Employing these photoanodes, solar water oxidation under near-infrared irradiation is demonstrated with an incident photon-to-current efficiency up to 25% at 770 nm illumination.

Bending Fracture Behavior and Chemical Degradation of Polymer-Passivated Flexible Organic Lead Halide Perovskite Thin Films

Although halide perovskites have significant potential for flexible electronic and optoelectronic devices, detailed bending mechanical behavior of encapsulated halide films has not been reported yet. Herein, we report the enhanced bending fracture behavior of flexible methylammonium lead triiodide (MAPbI3) thin films passivated with two selected polymer over-layers of polystyrene (PS) and poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The substantial mechanical enhancements were evident from the estimated fracture parameters, including critical strain, fracture energy, and film strength. For example, a highly concentrated PS passivation layer with ~51 nm thickness exhibited the largest fracture energy of 60.4 Jm-2 and the highest film strength of 299.9 MPa, which correspond respectively to ~96% and ~40% improvements compared to those of the non-passivated reference. Effects of the chemical passivation was also confirmed by chasing phase-dissolution of the perovskite with correlative changes of Hall-electrical properties.

Tailoring a Dynamic Metal–Polymer Interaction to Improve Catalyst Selectivity and Longevity in Hydrogenation

AbstractControlling metal–support interactions is important for tuning the catalytic properties of supported metal catalysts. Here, premade Pd particles are supported on stable polymers containing different ligating functionalities to control the metal–polymer interactions and their catalytic properties in industrially relevant acetylene partial hydrogenation. The polymers containing strongly ligating groups (e.g., Ar‐SH and Ar‐S‐Ar) can form a polymer overlayer on the Pd surface, which enables selective acetylene adsorption and partial hydrogenation to ethylene without deactivation. In contrast, polymers with weakly ligating groups (e.g., Ar‐O‐Ar) do not form an overlayer, resulting in non‐selective hydrogenation and fast deactivation, similar to Pd catalysts on conventional inorganic supports. The results imply that tuning the metal–polymer interactions via rational polymer design can provide an efficient way of synthesizing selective and stable catalysts for hydrogenation.

Tailoring a Dynamic Metal-Polymer Interaction to Improve Catalyst Selectivity and Longevity in Hydrogenation.

Controlling metal-support interactions is important for tuning the catalytic properties of supported metal catalysts. Here, premade Pd particles are supported on stable polymers containing different ligating functionalities to control the metal-polymer interactions and their catalytic properties in industrially relevant acetylene partial hydrogenation. The polymers containing strongly ligating groups (e.g., Ar-SH and Ar-S-Ar) can form a polymer overlayer on the Pd surface, which enables selective acetylene adsorption and partial hydrogenation to ethylene without deactivation. In contrast, polymers with weakly ligating groups (e.g., Ar-O-Ar) do not form an overlayer, resulting in non-selective hydrogenation and fast deactivation, similar to Pd catalysts on conventional inorganic supports. The results imply that tuning the metal-polymer interactions via rational polymer design can provide an efficient way of synthesizing selective and stable catalysts for hydrogenation.

Transient behavior in quasi-atomic layer etching of silicon dioxide and silicon nitride in fluorocarbon plasmas

The mechanism for atomic layer etching (ALE) typically consists of two sequential self-limited half-reactions—passivation and ion bombardment—which provide unique control over the process. Some of the possible benefits of this control include increased selectivity, reduced plasma induced damage, improved uniformity and aspect ratio independence. To achieve the greatest benefit from ALE, both half-reactions should be fully self-limited. In the experimental demonstration of ALE of SiO2 using fluorocarbon plasmas, the passivation phase typically consists of deposition of fluoropolymer on the SiO2 surface. This passivation step is not a self-limited reaction as the final polymer thickness depends on the passivation time. In this paper, results are presented from a computational investigation of the ALE of SiO2 and Si3N4 focusing on the implications of this nonself-limited passivation phase. The polymer overlayer was found to be critically important to the ALE performance, providing the main mechanism for selectivity between SiO2 and Si3N4. The polymer overlayer acts as a fuel for etching SiO2, which couples the etch depth per ALE cycle to the passivation time. Due to the inherently pulsed nature of the ALE mechanism, the polymer overlayer requires a finite number of cycles to reach a pulsed periodic steady-state thickness. Since the thickness of the polymer overlayer largely determines selectivity between SiO2 and Si3N4, the initial formation of an overlayer results in a transient period at the beginning of etching where high selectivity may not be achieved. For the etching of thin films, or applications which require very high selectivity, this transient etching period may be a limiting factor. Results are also presented using ALE to etch high aspect ratio self-aligned contacts which could not be cleared using continuous plasma etching with similar ion energies and flux ratios.

Detection of organic vapors on sputtered and annealed thin Au films

Unique optical properties of metal nanostructures enable construction of new types of chemical sensors. Nanostructures composed of Au on glass substrate were prepared by annealing of 2-20 nm thick sputtered Au films at 300 °C for 1 h. The annealing leads to transformation of the as sputtered continuous Au layers to a nanoisland structure. The forming nanostructure shows a strong, well defined surface plasmon resonance absorption band in UV-Vis spectrum, which is useful for construction of a chemical sensor. The samples were used to detect vapors of acetone and water in an experimental testing apparatus. The achieved signal-to-noise ratio was 583 and 386 for acetone and water vapors, respectively on the nanostructure prepared from 4 nm thick Au layer. The nanostructured sensitive layers, however, showed poor signal stability; therefore a polymer overlayer was introduced to protect it. The employed polystyrene film prepared by spin-coating improved sensitivity and selectivity of the sensor, while the dynamic properties of the sensing influenced only slightly.

Biosensing of Cholesterol and Glucose Facilitated by Cationic Polymer Overlayers on Ni(OH)2/NiOOH at Physiological pH

The electrocatalytic properties of the redox couple Ni(OH)2/NiOOH at physiological pH was investigated in the presence of cationic polymeric films providing alkaline microenvironments, with the objective of developing a biosensor for cholesterol and glucose. The Ni(OH)2/NiOOH modified electrodes in presence of the cationic polymer overlayers were well characterized by optical and electrochemical techniques. The modified electrodes showed excellent catalytic properties toward hydrogen peroxide oxidation and exhibited a wide linear range compared to the conventional and expensive Pt based catalysts used in H2O2 based sensors. The superb electrocatalysis of hydrogen peroxide could be combined with the enzymatic oxidation of cholesterol and glucose showing, the feasibility of monitoring biological metabolites like glucose and cholesterol using the basic catalyst Ni(OH)2/NiOOH at physiological pH. Modified electrodes were highly stable and yielded reproducible results. Further the influence of graphene layer in decreasing the overpotential of the detection of H2O2 and its effect on the biosensing of cholesterol and glucose was further investigated. The sensing protocol was validated in the analysis of blood sugar in serum samples.

Role of Interfacial Layers in Organic Solar Cells: Energy Level Pinning versus Phase Segregation.

PSS/interfacial layer was observed, the result was higher OPV device efficiencies. These results suggest that for each BHJ, the surface energies of the electrodes need to be carefully considered, as they will influence the local composition of the BHJ and resulting device performance.

RGB and white-emitting organic lasers on flexible glass.

Two formats of multiwavelength red, green and blue (RGB) laser on mechanically-flexible glass are demonstrated. In both cases, three all-organic, vertically-emitting distributed feedback (DFB) lasers are assembled onto a common ultra-thin glass membrane substrate and fully encapsulated by a thin polymer overlayer and an additional 50 µm-thick glass membrane in order to improve the performance. The first device format has the three DFB lasers sitting next to each other on the glass substrate. The DFB lasers are simultaneously excited by a single overlapping optical pump, emitting spatially separated red, green and blue laser output with individual thresholds of, respectively, 28 µJ/cm(2), 11 µJ/cm(2) and 32 µJ/cm(2) (for 5 ns pump pulses). The second device format has the three DFB lasers, respectively the red, green and blue laser, vertically stacked onto the flexible glass. This device format emits a white laser output for an optical pump fluence above 42 µJ/cm(2).

Moisture Distribution in Porous Oxide and Polymer Over-Layers and Critical Relative Humidity and Time of Wetness for Chloride and Non-Chloride-Bearing Atmospheres for Atmospheric Corrosion of Metals

Global Level of Connected Moisture (GLCM) and total moisture content (TMC) in porous over-layers on metals play a vital role in atmospheric corrosion. Using a model based on Kelvin condensation and Extended Hoshen-Kopelman algorithm these are computed as a function of relative humidity (RH) for 17 different over-layers with varying degrees of affinity to water. GLCM exhibits a critical RH (RH*) but TMC does not. The computed RH* agrees with the experimental result for iron for selected model parameters. RH* values are computed for several metal oxide and polymer over-layers on metals in chloride-bearing and non-chloride-bearing atmospheres. RH*of chloride-bearing atmospheres is found to be less than that of non-chloride-bearing atmospheres. A new formula is advanced for time of wetness using GLCM. Using RH and temperature data obtained from India Meteorological Department for three cities, time of wetness (TOW) is computed for the 17 systems. These results are compared with the predictions of a simplistic formula presently available.

(Keynote) Single Gold Nanowire Electrodes for Electrochemical Based Biosensing Applications

Compared to traditional macroelectrodes, nanoscale electrodes have tremendous potential as electrochemical sensors exhibiting enhanced performance. As critical dimensions of the electrodes enter the nano regime, 3D analyte diffusion profiles to the electrode dominate with a corresponding increase in mass transport, higher current densities giving rise to an increase in the ratio of faradaic to charging current, higher signal to noise ratios, steady-state sigmoidal voltammograms, low depletion of target molecules, low supporting electrolyte concentration requirements and shorter RC time constant 1-3. We and others have previously reported fabrication and in-depth electrochemical analysis of discrete gold nanowire electrodes for use in electrochemical applications. The single nanowire electrodes and nanowire electrode arrays were fabricated using a hybrid E-beam / photolithography approach, providing electrodes with well-defined and reproducible dimensions. Finite element diffusion domain simulation studies were employed to explore mass transport to nanowire electrodes. Simulation results suggested that radial diffusion to nanowires should be present at fast scan rates 4,5. This behavior was confirmed experimentally where CVs obtained in FcCOOH, were observed to be steady-state, with high currents (nA) and sigmoidal up to 50,000 mV s-1. The electrochemical responses of single and diffusionally independent arrays of nanowires, in model redox mediators, were excellently described by Butler-Volmer kinetics. To date we have employed these nanowire devices for highly sensitive detection of biochemical and chemical species including glucose and explosives 6-8. Herein we extend this work and explore the application of single on-chip gold nanowires for use as electrochemical biosensors with specificity provided by modifying the nanowire surface with polymer overlayers both with and without encapsulated biomolecules (application specific). We show highly sensitive dopamine with a measured limit of detection of 10 pM. We show real-time detection of glucose using an enzyme encapsulated within a polymer host without the need for a mediator. Finally we show label-free detection using electrochemical impedance spectroscopy of antibody-antigen binding events at a single nanowire electrode; see figure 1. Figure 1: Schematic showing the label-free detection of antibody-antigen binding at a single nanowire electrode References [1] Compton, R. G.; Wildgoose, G. G.; Rees, N. V.; Streeter, I.; et. Al., Chem. Phys. Lett. 2008, 458, 1. [2] Murray, R. W., Chem. Rev. 2008, 108, 2688. [3] Dawson, K.; Baudequin, M.; Sassiat, N.; Quinn, A.J.; O’Riordan, A. Electrochimica Acta 2013, 101, 169- [4] Dawson, K.; Wahl, A.; Murphy, R.; O’Riordan, A.,. The Journal of Physical Chemistry C 2012, 116 (27), 14665-14673. [5] Dawson, K.; Wahl, A.; Pescaglini, A.; Iacopino, D.; O'Riordan. A Journal of The Electrochemical Society 2014, 161 (2), B3049-B3054 [6] Dawson, K.; Strutwolf, J.; Rodgers, K. P.; Herzog, G.; Arrigan, D. W. M.; Quinn, A. J.; O’Riordan, A., Anal Chem 2011, 83, (14), 5535-5540. [7] Dawson, K.; Baudequin, M.; O'Riordan, A., Analyst 2011, 136, (21), 4507-4513 [8] Barry, S.; Dawson, K.; Correa, E.; Goodacre, R.; O'Riordan. A Faraday discussions 2013 164, 283-293 Acknowledgements This work was supported by Science Foundation Ireland under the US-Ireland programme (SFI12/US/I2476), by the Department of Agriculture Food and the Marine (13S405) and the Irish Higher Education Authority PRTLI programs (Cycle 3 “Nanoscience” and Cycle 4 “INSPIRE”). Figure 1

Effect of ageing and sterilization on plasma multilayer system

Single and coupled effects of sterilization (autoclaving and electron beam) and ageing of plasma polymer multilayer coating on polypropylene mesh are studied. The multilayer system consists in silver nanoparticles entrapped between two maleic anhydride plasma polymer layers and acts as a mechano-sensitive antibacterial reservoir. Tensile strength generates cracks within the plasma polymer overlayer which might be used as diffusive channels for Ag+ species. Changes in surface and bulk properties are followed by SEM, TEM, XPS and antibacterial assays. Whilst autoclave treatment influences surface structure unfavorably, electron beam irradiation with dose of 25 kGy does not induce surface physical or topographical changes. However, electron beam sterilization induces changes in chemistry of the surface causing a decrease in O/C elemental concentration ratio. The surface shows better antibacterial efficiency towards planktonic bacteria after electron beam sterilization and more surprisingly, the combination of electron beam sterilization and ageing has beneficial effect on final properties of the material under stretching.

Use of 3-aminopropyltriethoxysilane deposited from aqueous solution for surface modification of III-V materials

Focal plane arrays of strained layer superlattices (SLSs) composed of InAs/GaSb are excellent candidates for infrared imaging, but one key factor limiting their utility is the lack of a surface passivation technique capable of protecting the mesa sidewall from degradation. Along these lines, we demonstrate the use of aqueous 3-aminopropyl triethoxysilane (APTES) deposited as a surface functionalizing agent for subsequent polymer passivation on InAs and GaSb surfaces following a HCl/citric acid procedure to remove the conductive oxide In2O3. Using atomic force microscopy, variable angle spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and modeling with density functional theory (DFT), we demonstrate that APTES films can successfully be deposited on III-V substrates by spin coating and directly compare these films to those deposited on silicon substrates. The HCl/citric acid surface preparation treatment is particularly effective at removing In2O3 without the surface segregation of In oxides observed from use of HCl alone. However, HCl/citric acid surface treatment method does result in heavy oxidation of both Ga and Sb, accompanied by segregation of Ga oxide to the surface. Deposited APTES layer thickness did not depend on the substrate choice, and thicknesses between 1 and 20nm were obtained for APTES solution concentrations ranging from 0.1 to 2.5 vol%. XPS results for the N1s band of APTES showed that the content of ionic nitrogen was high (∼50%) for the thinnest films (∼1nm), and decreased with increasing film thickness. These results indicate that APTES can indeed be used to form a silane surface layer to cover III-V materials substrates. Such APTES silane layers may prove useful in surface passivation of these materials alone, or as surface functionalizing agents for subsequent covalent binding with polymer overlayers like polyimide.

Fabrication of sealed nanofluidic channels using site-selective direct write (maskless) X-ray lithography

Sealed nanofluidic channels with cross-sections as small as 60 nm × 60 nm were created in polymer bilayers using the focused X-rays of a scanning transmission X-ray microscope. These structures were then characterized by near-edge X-ray absorption fine structure spectromicroscopy, atomic force microscopy and scanning electron microscopy. The cross-sectional area of the nanochannels could be tuned by adjusting the area patterned in x and y and/or manipulating the bottom layer thickness. The maximum length was found to be limited by the efficiency of excavation of patterned material out of the channel, and the stability of the polymer overlayer which seals the channel. Schemes toward interfacing these nanochannels with conventional microfluidics are discussed.